Well Latias was apart of DPP when it was gen. According to bulbapedia you could find it Roaming Kanto in HeartGold and Pewter City in SoulSilver aparently. While these new move combos require something from Gen 10 to accomplish.
I think it's pretty obvious that adding an Uber to a tier that was locked for nearly a decade is substantively more disruptive than changing a few moveset legalities. Showdown is a Pokemon simulator (allegedly); we should be simulating the games first and foremost. Game changing, meta-warping mechanics changes like RBY body slam on normal types were implemented upon their discovery without much fuss. I'd argue it's actually
more important to allow tradeback movesets because that's
what is possible in the game. Suspecting things down like Latias is a player choice, Curse being possible on Scizor's moveset is just part of the actual real games now. Nintendo has in effect said that Gen 8 is not done yet, they could have chosen to force old moveset legalities upon import to Galar but they chose not to. There is no real reason to say moveset changes are some sacred cow that cannot be messed with when literally every other element is clearly unlocked after a generation ends.