That's not what I'm talking about. Those aren't universal effects. That's like saying the defining addition to Gen 8 was Aegislash's BST drop. I'm talking about fundamental changes to the battle system. Those are changes to specific moves, items, abilities, mons.Gen 8: Boots/Dex Cut/Teleport buff/Pursuit and HP removal
Gen 9; Recovery move nerf/Big moveset nerfs (Barra losing Flip Turn, Pex losing Knock, Blissey losing Teleport)
Special Split fundamentally changed how pokemon operated: pokemon with high special stats would no longer be offensive/devensive behemoths. Abilities gave almost every mon a a unique functionality, while the physical-special split gave every mon the ability to use coverage on its stronger stat, HA revamped abilities, giving a lot crazier ones than most normal abilities, while the introduction of Fairies changed the way the type chart works contributing to ghost/darks dominance in modern gens.
Recovery nerfs means that walls... do their job slightly worse.
Same goes for moveset nerfs.
Teleport, Pursuit & HP are all individual moves, same with HDB