They have always done something like this in a way. Wasn't Jirachi obtainable by having a save in Colosseum/XD (or was it Pokémon Box)? And I believe Manaphy was also obtainable by having a certain ammount of Pokémon deposited in Pokémon Ranch. And as a more recent example, we have Mew being obtainable by that weird Pokéball Joy Con thing in Let's Go, and Pikachu/Eevee's Gigantamax forms in Sword/Shield.
And those are the most notable examples, there's also stuff like some moves only being obtainable in certain Pokémon by having Stadium 2, such as Earthquake Gligar, and move-relearner exclusive moves.