Honestly for me what signaled the beginning of the end for the Simpsons was the movie. I genuinely didn't like it. Some of it felt like classic Simpsons, such as Albert Brooks as the insane EPA director.
"Sir, I'm afraid you've gone mad with power."
"Of course I have! Have you ever tried going mad without power? It's boring, no one listens to you."
That was literally the only joke in the entire movie that got a genuine laugh out of me and his whole performance was perfect in the role. That, at least felt like it had the classic Simpsons touch to it.
But the rest of it? Fucking stinksville. Everything from the overarching plot of Homer and his pigshit wrecking Springfield, the giant dome, the nuclear bombing, it all just felt so fucking meh to me. And a lot of the subplots, like Bart drinking and Lisa's inexplicably Irish love interest, were just completely uninteresting to me and I wanted them to resolve faster so they'd go away. The part where they went to Alaska was when I really started cringing.
The old Eskimo lady swinging her boobs around wasn't funny. The orchestral version of Spider-Pig was groanworthy and had me nearly cringing out my spine. Spider-Pig itself was an unfunny joke that too many people tried to force to exist longer than it ever should have. The joke about showing Bart's penis was just fucking juvenile and you could tell they were trying as hard as possible in some places to go beyond the show's normal humor and into Family Guy territory.
The entire thing was a massive letdown and it really feels like it would be representative of what The Simpsons would become. Mostly unfunny with the occasional chuckle and you wish it would just fucking end.