Not the first time. Latias and Latios aside, since they were in XY code, none of the new ORAS megas could interact with XY. Cosplay Pikachu was locked to the game as well, not even transferable to Bank.Tecnically, gen 7 broke this rule.
Sun/Moon are not able to obtain in any way the new Ultrabeasts added in USUM, nor Zeraora or UltraNecrozma forms.
In fact I remind there was a infamous issue with hackers somehow having managed to trade Zeraora before it's official release and it was possible to sent it to SM via Mystery gift, and it would break the game to those who'd receive it.
Even before that the assorted "third game" formes were softlocked out. Like they would gently revert if taken online, though you could still fight them locally. And of course NEP the pokemon SO locked away it couldnt even enter the trade room or be poketransported to gen 5