So why are people saying this is a 20 year convention that has been dropped?
Every new generation since Gen II has carried over all of the existing Pokémon from the previous one. With Gen III, some of the older Pokémon were absent for a while when Ruby and Sapphire came out, and since there wasn't compatibility with Gen II or Gen I games, they were unobtainable, for a while, until FR/LG came out. So it ended up, all of the Pokémon were available after all (of course they could be hacked even before the Kanto remakes came out).
Starting with Gen IV there's been direct compatibility with the previous generation, through different mechanisms / apps, all the way til Gen VII, which even included compatibility with Gen I and II through the Virtual Console of the 3DS.
So one thing would be that they told us, you can't bring your older Pokémon, like Piplup or something, but you can still catch a new one, in the new games, or future games. But the technological aspect isn't a problem anymore as it was with Gen III and II, so much that Pokémon Home will be able to bring all the Pokémon we've been collecting from Gen III - VII, but Sword and Shield will only be able to bring in the selection of Pokémon they decide to include in the Galar Regional Dex. There won't be a way to ever bring the cut Pokémon in, they most likely won't even be coded there. That's the first time in 20 years that this happens, a new main Pokémon game that doesn't bring back all the existing Pokémon introduced in previous generations, because they concluded that they can't handle that many Pokémon anymore. And SwSh won't be an isolated case, according to some of the statements they gave, that's the approach the series will take in future releases as well, so basically we can assume each upcoming game will have its own individual roster of Pokémon, limiting the connectivity between those versions to the use of only the Pokémon / attacks / abilities they have in common.
Regional Dexes were a form of limiting the available Pokémon at the beginning of the adventure. You would usually unlock the National Pokédex after entering the Hall of Fame or doing another specific task. As the series progressed, it lost more and more importance, to the point it wasn't even needed to be able to get/transfer in Pokémon that weren't in the Regional Dex of the game. And of course, the lowest point was SM/USUM where there wasn't even a National Pokédex at all (however, all the Pokémon were still able to appear there, via Poké Bank or lots of them being caught there). The National Pokédex was moved to the Pokémon Bank app then.
The National Pokédex is just a way to represent the movement, but the real issue here is that Pokémon non in the Galar Dex (thus, being part only of the National Dex) won't even exist in the new games. And that most likely be the case for every upcoming game in the future.