So for that late night Pokémon Sword/Shield rant I promised...
I think restricting the game to only the ones that appear in the regional Pokédex is not a great idea. The designers put themselves into a hole by having 800+ Pokémon and now not enough resources to model and animate them all for the new generation.
You can cite gameplay balance and development concerns for not including them all, but they simply should just be restricted from competitive play if that's the case.
If this is the trend they're going in to combat the growing amount of available Pokémon, then I think it's time for me to move on to other series. It seems hypocritical to play and get attached to Pokémon you've leveled up and trained for 15+ years only to not see it in the newest games based on seemingly random factors or preferences by the developers.