Originally Posted by
Adramalech
Bulbasaur, Squirtle, Mewtwo, Mew, Celebi, Jirachi, Cobalion, Terrakion, Virizion, Reshiram, Zekrom, Kyurem, Keldeo, Rowlet, Litten, Popplio, Cosmog, Necrozma, Marshadow, Zeraora, Meltan, and all evolutions and forms related to these pokemon (except Necrozma-U) are in the game files, marked as "Foreign" for Pokedex purposes. We know Mew is obtainable, so it's unknown if the others will be available somehow or not. They also have full compatibility with new TMs and TRs, and seem to be fully prepared to be playable.
Whether they will be available at some point or not, it's unknown. But their data is there. I'm guessing either they'll be made available through raids, or some other type of promotion or some crap. I don't see the starter lines being behind Eternatus in the Pokedex for no reason...