One region must be completely removed from existence. Which do you delete?

I meant the Pokémon that were introduced in Johto

Yeah but it also removes all characters. This includes Elite Four and Champion so you'd be removing a Kanto Gym Leader and two Elite Four members for Kanto. You'd also be removing their Pokemon(which uses a lot of Kanto) so that's why if you remove Johto, you're removing a good chunk of Kanto while doing so since they're intertwined. Why else does Route 26 exist.

Completely gone

All characters

All Pokemon

All the lore

And the gimmick

Poof

Pick wisely

That implies everything. As I said Johto and Kanto are heavily intertwined. Removing one seriously harms the other. Not even sure you could have them as separate options because if you remove Kanto, then what Pokemon would Johto use? Faulkner and Bugsy use Kanto Pokemon. Whitney would just have Miltank(probably enough), Morty wouldn't have anything, Ditto Chuck. Jasmine just has Steelix but how would she evolve or get one if Onix doesn't exist? Pryce would just have Piloswine and Clair would just have Kingdra but how would she get it if Horsea/Seadra is wiped out?

I don't really care about johto and hoenn, but that's probably I never saw a anime-episode of it