On 11th June 2019 it was announced that starting with Pokémon Sword and Pokémon Shield that the games will not have every Pokémon in them from then on. Because of that #BringBackNationalDex was made.
https://www.youtube.com/watch?v=TmWu-f6L0Mo&t=1624sIt has now been five years since then. Five years of not having every Pokémon. As of now Pokémon can not be in any Nintendo Switch main game if they are of any of these species:
Patrat
Watchog
Pansage
Simisage
Pansear
Simisear
Panpour
Simipour
Furfrou (Soon to be included in the next game.)
Remember that every Pokémon is someone's favourite. We can catch 'em all but they can only all be together in Pokémon Home.
https://bulbapedia.bulbagarden.net/wiki/List_of_Pok%C3%A9mon_by_availability 