Hi
There are certain event Pokémons from older generations that can have a move that is unobtainable for that Pokémon by any other means.
For example Petal Dance Meowth
The other day I was building a 9th gen team in Pokémon Showdown and Persian with Petal Dance was not allowed. Only 8th gen and prior gens allows for these moves to stay with the Pokémon.
That is a bummer tbh
Is there any post or page from gf or other site about this matter?