A thought came to me regarding the fifth bullet point in the below graphic, regarding moves on HOME visitors:
Specifically, it mentions that if HOME visitors know moves that aren't usable in Champions, you have to replace them in the latter. Presumably this is mainly to account for things like Bank-transferred Pokémon that still know moves such as Return or Sky Drop that GameFreak probably never wants to add back in. However, what of things such as event-exclusive moves that
are usable in Champions? As an example, in Gen 8 there was a distribution for a Pikachu that knows Sing. In the near-guaranteed event that Sing is a usable move in Champions, will bringing the Pikachu knowing Sing into Champions let it use Sing in battles? Given the wording of the bullet point specifically referring to unusable moves, it would suggest so.
This would also extend beyond event moves and into differing learnsets between games, such as a Gardevoir knowing Ice Beam which is a move it can only learn in LA. Because of this, I'm expecting Champions to internally define legal moves for each individual Pokémon so that even moves usable in Champions will be flagged as needing to be replaced if they don't appear on its legal list. In my Pikachu example, the game would alert the player that Sing needs to be replaced, despite Sing being usable by something like Lapras. I know the bullet point doesn't suggest such restrictions, but I think it would be way too convoluted and disadvantaging otherwise. Not to mention that having internally defined legal moves would also allow for revamps between seasons if they decide Pokémon need moveset buffs or nerfs, which is something that GameFreak already likes to do between generations.