You did valid good points in this post but I have some problems with some things you assumed. Notably, you haven't done a dynamic approach on the meta which could have been interesting to show our differences on the subject. I will try to argue about our different point of views in the matter in trying to give a global approach.
I will first talk about the part 2 because it is important to understand the part 1.
Shiinotic is the definition of a niche mon which becomes suddendly good only due to only one mon. In the beginning of march, when Shiftry started to rise ranks, Shiinotic was in the C rank on the viability rankings. It was deserved, this Pokemon is outclassed almost totally by Gourgeist-Super which is far bulkier (On Golem Stone Edge Gourg-S takes max 33%, Shiinotic max 51%). In this aspect, it can be compared to a lower extent to Seaking or Magneton which were only used at the beginning due to Frosttom or Scizor respectively.
On the same kind of idea, Mawile was B- before Shiftry rose. It is nominated to be A- today. This kind of thing is okay in itself if the mons used have some merits and doesn't make your team worse. It's however the case.
The restriction of the teambuilding that Shiftry exerces puts you in a disadvantaged situation since it is picky in terms of checks and counters. Even when it is not in teams.
Example 1 : DurzaOffTopic versus Xayah ZUPL :
https://replay.pokemonshowdown.com/gen7zu-960390659
DurzaOffTopic took a team which was 6-0 by Shiftry. He fished for the match up and it was apparently the good decision since Xayah didn't bring Shiftry. Xayah however has been prepared to Shiftry in taking Shiinotic.
Turn 1, Durza leads with Komala band. Xayah doesn't know the set and decides to pivot with u-turn. The damages revealed that it's probably a band or an assault vest Komala. Xayah is already in a difficult position and has to decide between Shiinotic/Metang. He lost the fifty fifty and takes 70% on Shiinotic. Shiinotic is 2HKO by Electivire some turns later by Wild Charge which makes Evire and Komala band free to do damages.
Let's imagine a world where Shiftry doesn't exist. The team of Durza won't have changed much since his team doesn't handle Shiftry. Xayah would have certainly taken Gourgeist-Super which, outside of the Shiftry match up, is better than Shiinotic. The game would have been very different, Komala couldn't more or less got rid of Shiinotic turn 2. He would have got a more solid electivire and komala band answer (two A+ mons) which wouldn't force him to start the game with an impossible come back. Maybe Xayah would have won in this tournament battle, maybe not. One thing is sure, the presence of Shiftry in the meta substantially weakens the team of Xayah.
Example 2 : Tsareentje versus a fruitshopowner ZUPL :
https://replay.pokemonshowdown.com/gen7zu-959796515
Both teams are prepared against Shiftry. Once has Vullaby (C- mon) the other Silvally Dark.
The team of fruitshop owner is interesting because he used the lure Poison Jab Sandslash to hurt Shiinotic and a curse lickilicky which is a really dangerous wincondition. However, his match up is horrible. Notably, Leafeon, a mon with the exact same coverage than Shiftry, 6-0 his team. If Tsareentje used Swords Dance turn one, the game would have been over.
The team of fruitshop wasn't bad by any means. It was a good antimeta team. He used Vally-Dark against Shiftry, which is a decent shiftry answer in the sense where it is not killed by Sucker Punch at the +2 and can u-turn on it pretty safely since it's faster. However, it's absolutely not a grass anwer. He choosed to be safe against a S rank mon instead of a B+ one which is logical but cost him the victory.
With these examples, we can conclude several things :
1) Shiftry, even when not present in battles, has a clear influence on the teambuilding.
This can also be seen in the usage where Mawile is regulary in the top 10 of usage in tournament, notably in the week 2 and 4 of the zupl where it is the sixth mon the most used (I remind that it was a B- mon before Shiftry becomes played). Shiinotic, a pokemon almost totally outclassed by Gourgeist-Super outside of the Shiftry match up, is most used in zupl than its counterpart. It's especially true when we remove stall where Shiinotic has almost a monopoly of use instead of Gourgeist-Super. The same can be said about Altaria Z-move which was rarely seen before and in a lesser extent the high usage of Silvally. This kind of thing is a strong centralization. You can call that a normal centralization or an overcentralization but you cannot deny that the influence of Shiftry on the teambuilding is far more important than the influence of any other pokemon played in ZU.
2) Since Shiftry is picky in answers, the teams which are built to handle Shiftry put you in a disadvantaged position against other threats.
It is not something common. Others top tier have reliable and viable answers. Combusken is answered by viable mons and very used such as Mareanie, Swanna, Pyukumuku, Silvally-Water. Electivire and Golem have normally a lot of good answers such as Gourgeist-Super, Bronzor, Bulky grounds such as Sandslash, and bulky grass overall (even leafeon is a soft counter). The only other top tier mon, without being a wallbreaker, which doesn't have reliable viable answers is Swanna which is also a mon pretty busted in ZU.
3) The other threats and top tiers benefit of a weaken metagame.
When players play Altaria, Mawile, Shiinotic and Silvally to handle Shiftry well, they are limited by the number of slots by team since they have to handle as well mons such as Combusken, Swanna,to pressure stall, to beat toxic spikes, etc. What I mean is they don't usually have the slot for an other physical sponge. If you take that into consideration, the relatively strength of Electivire, Golem and mostly Leafeon and Komala in the last tournament is easy to explain.
When you take Mawile or Altaria-Zmove because Shiftry, it's obvious that Evire and Golem become more threating than usual. It's exactly the same when you trade Gourgeist-Super for Shiinotic where you use a golem switch in which is 2HKO (!) by Golem after rocks and takes 45% on Ice punch from evire.
In the case of Leafeon, it just kills every team which relies on Silvally to handle Shiftry. More than that, with normalium-Z which is a standard set, it one shots shiinotic at +2 and it's a roll for altaria. The funniest part is it has the exact same coverage than Shiftry but a team which handles Shiftry doesn't necessary handle Leafeon. It exists good answers to Leafeon such as Gourgeist-Super or faster mon able to revenge kill common versions such as Simisear, Pinsir scarf, Rotom-fan scarf, toucannon scarf, etc. The majority of the them are killed by SP at +2 so you don't use them to handle Shiftry. I think I don't have to explain about Komala, the no-presence of Gourgeist-Super and Avalugg outside of stall is a good thing.
And it becomes absurd. Why bronzor, a mon which counters most golem variant, is one of the best swanna answer, is one of the few good rockers, is correct against evire, sucks if it is not due to Shiftry ? Why Leafeon, a mon which is played in a meta when every team should carry a good grass/dark resist, manages, in being played more than 10 times, to reach 75% winrate ? Tell me if it is not because the majority of shiftry answers are bad ? Why people still play Shiinotic instead of Gourgeist-Super even if they are aware that komala band/bouffalant/leafeon or even bibarel lol destroy their team ?
If you want a single reason why Shiftry should be suspected, here we go :
Shiftry should be suspected because it makes ZU less competitive
With the unhealthy impact Shiftry exerces on the metagame, in decreasing the viability of in theory good mons such as Gourgeist-Super, Bronzor, Silvally-Water, and in forcing players to play suboptimal mons such as shiinotic, mawile, or vallys in grass checks, Shiftry makes other top tiers and threats better than they actually are. It restricts the teambuilding around its use and force players to be threaten by top tiers.
It's normal to don't cover perfectly any mon in the tier. It is not normal to create classic teams 6-0 by top tiers/good mons. It's especially true for balances and offenses which are the playstyles the most restricted by the presence of Shiftry. They are forced to play bad mons or have trouble to deal with shiftry in itself.
The ZU is less competitive than before because it is nowadays too much based on the match up. You have to pick against which good mons you lose. You start a game in hoping you covered the right mon your opponent used. It's just frustrating because it's something which is not always in your power, especially when you don't know the opponent. Anyway a competitive game shouldn't rely too much on the good card you pick. I would say that one battle on two is won at the team preview. Personally, it's not a metagame I appreciate playing. I'm just exhausted when I lose against a mon I don't have any counter due to the nature of my team, I have no pleasure to beat an opponent because he didn't have any answer to one of my mon. The state of ZU right now is almost worse than the one when Rotom-Frost existed. At least, during the frosttom area, you had to win 50/50 during the battle, not before the battle in choosing your team.
Shiftry is the main reason, it's the mon which restricts the most your teambuilding and which force you to play unreliable mons instead of viable one. Since it's the third time people asked for the suspect, I think it's the least of it. I'm obviously in favour of a ban. Nothing except bans could fix the tier. I hope something will be done, at least to make the tier more competitive but also more enjoyable to play.
(* I apologize for the English mistake it is late. Also, even if answered to Hoen, it's more a general answer. He doesn't have to feel specially targeted).
I will first talk about the part 2 because it is important to understand the part 1.
About defensive answers, Silvally-Fighting, Dark and Mawile take 50% of their life on Leaf Storm. The only version of Altaria which tanks Shiftry is the Z-move 252/160+ in defense which doesn't have to take stealth rocks to be sure to don't die on SP at +2.Shiftry is not broken (Part 2):
The second bullet point argues that a broken mon is one, "whose only counters or checks are extraordinarily niche Pokemon that would put the team at a large disadvantage elsewhere."
Shiftry, in both ZU ladder usage, tour usage, and viability rankings has both naturally viable checks on top of extra niche ones. Niche examples include Vullaby and Zweilous, which can work for checking Shiftry and Egg but not much else for a team. However, if the following S to B- ranked mons are proven to be checks that are natural to the ZU metagame and provide advantages elsewhere than just checking Shiftry, then this definition of broken must be ruled out as well for Shiftry.
Defensive:
Offensive:Silvally-Fighting, from the analysis: "Speed tier, adequate offense, good coverage, and access to Defog and U-turn make it a very solid pick for many teams in ZU." These traits alone, along with more appraisal from the analysis, show that this mon exists as much more than a Shiftry check.
Silvally-Dark is one of the few STAB Pursuit user in the tier. That alone makes it a prime pick for supporting Psychics like Egg and Beh to take out Bronz, and also for general Pursuit use. Like Shiftry, the Dark STAB is rare here, so even SD Multi Attack is threatening and unique. Lastly, a BoltBeam set on a Silvally with the Dark typing works against multiple MUs, including checking Swan and Egg.
Maw is a consistent Intimidate user and Normal/Flying resist for the tier that can also provide SR. Maw also checks and punishes the constant U-Turn spam from the Silvally forms thanks to its resistance and sometimes Rocky Helmet.
Phys Def Alt, again from the analysis, "is one of the premier Defog users in the tier. It differentiates itself from other entry hazard removers such as Silvally-Water, Komala, Silvally-Fighting, and Swanna through its combination of resistances, ability in Natural Cure, outstanding overall bulk, and access to reliable recovery." The recent trend with Alt is to run Firium Z instead of leftovers to check Shift on stall team, but ultimately the set and mon still achieves the previously mentioned purposes on top of checking Shift.
Shii resists both of Shifts stabs and only fears bait sets or Flyium Z move sets, which in turn make Shiftry less threatening as a whole. Past that, Shii has a great movepool with Spore and Strength Sap and can stall out many physical attackers like Golem or Evire.
Eviolite Combusken can afford to SD in the face of Shiftry with a safe switch in and tier teams apart; worthy of an A+ rank mon. Even without SD or Eviolite, a healthy Combusk can fire off an attack without needing to protect against Shiftry.
Z Mirror Move Swanna dodges the sucker punch and allows the player to set up and KO a weakened Shiftry at around 50% health. Past that, all healthy Swannas can still survive an LO sucker without rocks, or a 37% roll with them. Z Move Shifts cannot 1hko with sucker after rocks, doing only 58% max.
Grassium Z Bellosum and SD Jump both work to Strength Sap and set up in the face of Shift, but also be great late game sweepers on there own.
These sets mostly were concerned with the standard LO Mixed Shift, which was one of the more used and successful Shifts from the ZUPL data. Z move sets or special NP sets either are easier to deal with or have overlapping defensive or offensive checks.
Again, if one were to say that these mons can be checks to Shiftry, and be viable even outside the Shiftry context, then it is fair to say that Shiftry is not broken by having only niche checks. These mons prove that Shiftry can be punished in multiple ways, and that the "Shift check" in teambuilding is natural to other teambuilding roles.
Shiinotic is the definition of a niche mon which becomes suddendly good only due to only one mon. In the beginning of march, when Shiftry started to rise ranks, Shiinotic was in the C rank on the viability rankings. It was deserved, this Pokemon is outclassed almost totally by Gourgeist-Super which is far bulkier (On Golem Stone Edge Gourg-S takes max 33%, Shiinotic max 51%). In this aspect, it can be compared to a lower extent to Seaking or Magneton which were only used at the beginning due to Frosttom or Scizor respectively.
On the same kind of idea, Mawile was B- before Shiftry rose. It is nominated to be A- today. This kind of thing is okay in itself if the mons used have some merits and doesn't make your team worse. It's however the case.
The restriction of the teambuilding that Shiftry exerces puts you in a disadvantaged situation since it is picky in terms of checks and counters. Even when it is not in teams.
Example 1 : DurzaOffTopic versus Xayah ZUPL :
https://replay.pokemonshowdown.com/gen7zu-960390659
DurzaOffTopic took a team which was 6-0 by Shiftry. He fished for the match up and it was apparently the good decision since Xayah didn't bring Shiftry. Xayah however has been prepared to Shiftry in taking Shiinotic.
Turn 1, Durza leads with Komala band. Xayah doesn't know the set and decides to pivot with u-turn. The damages revealed that it's probably a band or an assault vest Komala. Xayah is already in a difficult position and has to decide between Shiinotic/Metang. He lost the fifty fifty and takes 70% on Shiinotic. Shiinotic is 2HKO by Electivire some turns later by Wild Charge which makes Evire and Komala band free to do damages.
Let's imagine a world where Shiftry doesn't exist. The team of Durza won't have changed much since his team doesn't handle Shiftry. Xayah would have certainly taken Gourgeist-Super which, outside of the Shiftry match up, is better than Shiinotic. The game would have been very different, Komala couldn't more or less got rid of Shiinotic turn 2. He would have got a more solid electivire and komala band answer (two A+ mons) which wouldn't force him to start the game with an impossible come back. Maybe Xayah would have won in this tournament battle, maybe not. One thing is sure, the presence of Shiftry in the meta substantially weakens the team of Xayah.
Example 2 : Tsareentje versus a fruitshopowner ZUPL :
https://replay.pokemonshowdown.com/gen7zu-959796515
Both teams are prepared against Shiftry. Once has Vullaby (C- mon) the other Silvally Dark.
The team of fruitshop owner is interesting because he used the lure Poison Jab Sandslash to hurt Shiinotic and a curse lickilicky which is a really dangerous wincondition. However, his match up is horrible. Notably, Leafeon, a mon with the exact same coverage than Shiftry, 6-0 his team. If Tsareentje used Swords Dance turn one, the game would have been over.
The team of fruitshop wasn't bad by any means. It was a good antimeta team. He used Vally-Dark against Shiftry, which is a decent shiftry answer in the sense where it is not killed by Sucker Punch at the +2 and can u-turn on it pretty safely since it's faster. However, it's absolutely not a grass anwer. He choosed to be safe against a S rank mon instead of a B+ one which is logical but cost him the victory.
With these examples, we can conclude several things :
1) Shiftry, even when not present in battles, has a clear influence on the teambuilding.
This can also be seen in the usage where Mawile is regulary in the top 10 of usage in tournament, notably in the week 2 and 4 of the zupl where it is the sixth mon the most used (I remind that it was a B- mon before Shiftry becomes played). Shiinotic, a pokemon almost totally outclassed by Gourgeist-Super outside of the Shiftry match up, is most used in zupl than its counterpart. It's especially true when we remove stall where Shiinotic has almost a monopoly of use instead of Gourgeist-Super. The same can be said about Altaria Z-move which was rarely seen before and in a lesser extent the high usage of Silvally. This kind of thing is a strong centralization. You can call that a normal centralization or an overcentralization but you cannot deny that the influence of Shiftry on the teambuilding is far more important than the influence of any other pokemon played in ZU.
2) Since Shiftry is picky in answers, the teams which are built to handle Shiftry put you in a disadvantaged position against other threats.
It is not something common. Others top tier have reliable and viable answers. Combusken is answered by viable mons and very used such as Mareanie, Swanna, Pyukumuku, Silvally-Water. Electivire and Golem have normally a lot of good answers such as Gourgeist-Super, Bronzor, Bulky grounds such as Sandslash, and bulky grass overall (even leafeon is a soft counter). The only other top tier mon, without being a wallbreaker, which doesn't have reliable viable answers is Swanna which is also a mon pretty busted in ZU.
3) The other threats and top tiers benefit of a weaken metagame.
When players play Altaria, Mawile, Shiinotic and Silvally to handle Shiftry well, they are limited by the number of slots by team since they have to handle as well mons such as Combusken, Swanna,to pressure stall, to beat toxic spikes, etc. What I mean is they don't usually have the slot for an other physical sponge. If you take that into consideration, the relatively strength of Electivire, Golem and mostly Leafeon and Komala in the last tournament is easy to explain.
When you take Mawile or Altaria-Zmove because Shiftry, it's obvious that Evire and Golem become more threating than usual. It's exactly the same when you trade Gourgeist-Super for Shiinotic where you use a golem switch in which is 2HKO (!) by Golem after rocks and takes 45% on Ice punch from evire.
In the case of Leafeon, it just kills every team which relies on Silvally to handle Shiftry. More than that, with normalium-Z which is a standard set, it one shots shiinotic at +2 and it's a roll for altaria. The funniest part is it has the exact same coverage than Shiftry but a team which handles Shiftry doesn't necessary handle Leafeon. It exists good answers to Leafeon such as Gourgeist-Super or faster mon able to revenge kill common versions such as Simisear, Pinsir scarf, Rotom-fan scarf, toucannon scarf, etc. The majority of the them are killed by SP at +2 so you don't use them to handle Shiftry. I think I don't have to explain about Komala, the no-presence of Gourgeist-Super and Avalugg outside of stall is a good thing.
So we can now start to talk about the winrate of Shiftry in ZUPL. We had a tournament which was very prepared against the Shiftry presence (see once again the mawile, shiinotic, altaria z-move and silvally usage). It shows that it is possible to make teams in ZU which deals with Shiftry comfortably. Nevertheless, despite of heavy preparation, Shiftry still manages to reach almost a 50% winrate which is relatively crazy to be honest. It is not the only thing, it also shows that a lot of teams, whose for a large part are prepared against Shiftry, have currently a bad match up against other top tiers.
And it becomes absurd. Why bronzor, a mon which counters most golem variant, is one of the best swanna answer, is one of the few good rockers, is correct against evire, sucks if it is not due to Shiftry ? Why Leafeon, a mon which is played in a meta when every team should carry a good grass/dark resist, manages, in being played more than 10 times, to reach 75% winrate ? Tell me if it is not because the majority of shiftry answers are bad ? Why people still play Shiinotic instead of Gourgeist-Super even if they are aware that komala band/bouffalant/leafeon or even bibarel lol destroy their team ?
If you want a single reason why Shiftry should be suspected, here we go :
Shiftry should be suspected because it makes ZU less competitive
With the unhealthy impact Shiftry exerces on the metagame, in decreasing the viability of in theory good mons such as Gourgeist-Super, Bronzor, Silvally-Water, and in forcing players to play suboptimal mons such as shiinotic, mawile, or vallys in grass checks, Shiftry makes other top tiers and threats better than they actually are. It restricts the teambuilding around its use and force players to be threaten by top tiers.
It's normal to don't cover perfectly any mon in the tier. It is not normal to create classic teams 6-0 by top tiers/good mons. It's especially true for balances and offenses which are the playstyles the most restricted by the presence of Shiftry. They are forced to play bad mons or have trouble to deal with shiftry in itself.
The ZU is less competitive than before because it is nowadays too much based on the match up. You have to pick against which good mons you lose. You start a game in hoping you covered the right mon your opponent used. It's just frustrating because it's something which is not always in your power, especially when you don't know the opponent. Anyway a competitive game shouldn't rely too much on the good card you pick. I would say that one battle on two is won at the team preview. Personally, it's not a metagame I appreciate playing. I'm just exhausted when I lose against a mon I don't have any counter due to the nature of my team, I have no pleasure to beat an opponent because he didn't have any answer to one of my mon. The state of ZU right now is almost worse than the one when Rotom-Frost existed. At least, during the frosttom area, you had to win 50/50 during the battle, not before the battle in choosing your team.
Shiftry is the main reason, it's the mon which restricts the most your teambuilding and which force you to play unreliable mons instead of viable one. Since it's the third time people asked for the suspect, I think it's the least of it. I'm obviously in favour of a ban. Nothing except bans could fix the tier. I hope something will be done, at least to make the tier more competitive but also more enjoyable to play.
(* I apologize for the English mistake it is late. Also, even if answered to Hoen, it's more a general answer. He doesn't have to feel specially targeted).
(Water, Dragon)
and a few other more niche ones like Komala, Kadabra, Simisage etc.
and again a few niche ones like Jumpluff, Electrode, Purugly, and Gourgeist-Small.
My opinion only here