Trade evolutions. They kill the playability of older games, especially after the online services get discontinued. I don't have a second system, nor do I know anyone else irl who plays either, so I probably won't be able to get any more trade evolutions in my older games (White 2, Y, Sun, Ultra Moon). No more Gengar or Machamp for me, I guess. I also think of people playing the games on emulators- I don't know if trade is even possible on emulators (aside from in-game trades ofc). I like to *try to* complete the Pokédex, so I guess that's the angle I have on this- if you don't have a way to connect with another player, you'll never be able to complete the Pokédex because of trade evolutions. I think there's something in the Switch games called the Linking Cord that can trigger trade evolutions, but I don't have any of those so this doesn't really apply to me.
(Also somewhat related to this, but side story: as a kid, I raised a Haunter to level 100 and didn't know why it didn't evolve. My sister and I were trading with each other and one of us happened to send over a Kadabra and it evolved, so we kept trading other mons back and forth to see if they would evolve. I traded over the Haunter, and it evolved)