I like this. It's not on the horizon yet, but there will be future Pokemon games, and if the devs don't bow to fan outrage, we may not see the National Dex again. Right now we have Gen 1-7 and Gen 8 with separate "sets" of Pokemon you're allowed to have on cartridge, but this might get more complicated with each new generation.
A more elegant and logically consistent way of phrasing this could be to say that any Pokemon that was legally obtainable at any point in a main series game, would be allowed in NatDex. Off the top of my head this would make alternate forms like Cosplay Pikachu, LGPE Pikachu/Eevee and Spiky-Eared Pichu (lol) available, as well as previous abilities like Levitate Gengar/Flash Fire Entei etc and a few obscure Gen 2 event moves (Rapid Spin Golem, Encore Phanpy, Petal Dance on a bunch of stuff...). Of course, if you wanted to run Rapid Spin Golem or Levitate Gengar, it could only use its Gen 2 and Gen 3-6 moves respectively.
This would make move legality a bit confusing possibly, but it'd be more consistent.