Are Pokémon still legal when moved back for Gen 5 online play?

[ArtemisWinter]

I understand that Pokémon are modified and shouldn't be used in online play when transferred to modern games, but is this also the case for Pokémon in non-modern games?

Gen 5 isn't considered a modern Pokémon game, but online play is still possible. So if I move a Gen 5 Pokémon into OpenHome, then later move them back to my Gen 5 save file, would those still be considered legal to use online?

I guess the root of my question is to better understand how Pokémon are modified by OpenHome; didn't want to just assume anything.