Landorus-I has been a looming threat in the Monotype metagame ever since the release of the Crown Tundra DLC. The combination of its amazing movepool, solid base 115 Special Attack, and Sheer Force paired with Life Orb makes it an incredibly powerful wallbreaker that's troublesome to deal with for the majority of the metagame. Outside of its standard attacks such as Earth Power, Sludge Wave, and Focus Blast, Landorus-I can run moves to punish many of its switch ins such as Knock Off for the likes of Slowking and Latias, Rock Slide for Flying-types such as Tornadus-T and Zapdos, and Gravity which allows it to beat Pokemon such as Celesteela. The removal of Hidden Power in this generation provides less counterplay to Landorus-I and its amazing support, especially on Flying-type teams, has made it a much bigger threat than in previous generations. Due to its power, unpredictability, team support, and restriction of a number of types, the council has decided to ban Landorus-I. Some time after MWP is over we will revisit its ban and likely hold a suspect test.
Tagging The Immortal & Kris to implement, thanks!