This is just a part of a bigger trend of GF cutting features with no good reason. It's absolutely the worst of them, but here is everything else in list form:
Yeah, and here is where I disagree with you and the majority of the Pokémon fanbase. Not every new entry has to have every single feature from every single game... that's not how games are supposed to be designed, really. I've always hated how Pokémon prioritised this cumulative approach to sequels, where nothing could ever be dropped, because inevitably it means that every single feature that gets added in, will now be a part of the games forever and if I don't like it then it will significantly change my feelings on the franchise as a whole rather than a single game. This happened with Sun/Moon, where I didn't like the inclusion of Alola forms, Z-Moves, Ultra Beasts and Ash-Greninja, and instead of just going "okay I'll try to get in the mood for these things that are in this specific game" it's more like "oh well I guess now the entire Pokémon franchise will forever be riddled with these things". In that sense, I'm kinda glad they're finally taking an open approach to cutting features because now it means each game can only be judged on its own merits.
Think about it, no other game works like that. You don't see people complaining that Majora's Mask dropped Ocarina of Time's adult Link. Can you imagine if the Zelda franchise worked this way? The next Zelda game wouldn't only be judged on its exploration, worldbuilding, story, dungeons and all that stuff, it'd also be judged on whether it included Wolf Link, Painting Link, motion controls, touch controls, and more, and also how well it brought those things over without them being "half-assed".
It's just not a good philosophy to have, it never was. I understand that people have become used to it with Pokémon, but at some point it needs to stop. Seasons were not a good feature and didn't add anything to the game other than forcing you to wait months to get stuff you want. Secret Bases were an alright feature in gen 3 but there's no reason they had to become a staple of the series. Rotation battles were just an annoying gimmick that brought nothing interesting to the table, and to a lesser extent the same applied to Triple battles. These things don't need to be in every game once they're introduced, there's just no reason to.
Of course, that doesn't mean it's alright to cut any feature. Gen 3 cutting the day-and-night system, gen 7 cutting the PSS (which you somehow forgot to mention), and gen 8 cutting full Pokémon compatibility were all baffling choices that felt like an actual step back rather than just some random feature they didn't bother to include. These were, and are, not okay.
I just hate this notion that each and every little thing that doesn't make it to a new game is a terrible thing of Game Freak to do. It's a freedom-limiting kind of pressure that no other developer gets. Not that Game Freak does wonders with freedom.