For me, the crazy everything-is-a-dupe drove me away. Same as so many others mentioning here.
But a lot of other things were also wrong and they are worth listing too:
I stopped looking at the answers as they aged. So many of the top or accepted answers were quickly out of date (but you couldn’t ask again because or the dupe mods).
A lot of the accepted or top answers had always been wrong even when written. If you knew more about something you could spot this but it was pointless to engage and try and change it. Popularity is not about being correct.
There was no need for all the other sites. It would have been better with one inclusive site. So often a good question would be asked and mods would comment it should be asked on some niche site with no traction and so died the question.
The gamification point system favoured the early people like me. I probably got most of my points long after I had actually moved on because my top answers were still racking up votes. There was no hope for a more recent joiner (even joining 10 years ago) to ever catch up.
And the hardest most interesting questions got no answers. If you were truly stumped on a difficult algorithm then it really wasn’t the place to ask.
That’s some of what I remember being wrong with SO long before LLMs were mainstream.
I shudder to think that LLMs learn from SO answers. It would explain so much of the worst vibe coding though :)