I'm not sure what it is, and I don't really care to think about it anymore. Plenty of people have said their peace and given their reasons why the game feels worse. Less individual player agency, games feeling shorter, every champ is a "burst" champ, mobility creep, abhorrent matchmaking full of smurfs, boosters, boosted accounts, duo abusing to boost etc. Preseason feels even worse, as it doesn't seem to have addressed any of these issues.
All I know is, I'm having less fun than ever playing this game. And even after taking a few months off only playing a couple games here and there, it doesn't seem to have gotten better. My next break might just be my last. I'll probably keep up with the game from the pro scene, but playing the game just feels bad, and has for a while.