Now that we're getting a full list of all the Pokemon in the game, and thus a list of Pokemon NOT in the game, here's a list of Pokemon that have only showed up in the regional Pokedex of their debut generation.
>https://bulbapedia.bulbagarden.net/wiki/User:SilSinn9801/List_of_Pok%C3%A9mon_that_appear_in_only_one_regional_Pok%C3%A9dex

If a Pokemon you like has been cut from SW/SH, it's entirely possible you're not going to see them again until the generation they're from gets remade. If a Pokemon you like IS in SW/SH, this is likely the last time they'll be playable for at least 5 years at the absolute lowest.