It's not stuff, it's just the nostalgiacigarette "90s kids" who act like it became the worst thing ever after the original creator left. I think it actually got better, they could tone down the squidward abuse research though.
I don't know, the original writers quit or were going to quit after the first movie cause they kinda figured that'd be the end of it.
Then they had to replace them and find a new direction for the show.
I feel like the quality did decline, not that there aren't a few good episodes after the movie though.
I don't actually watch the show obviously, but I've watched a good amount of reviews.