StackOverflow is still pretty good.
I believe the parent commenter did not mean that StackOverflow itself became bad, but rather that its content gets copied and cloned by bots and posted as much lower quality content on Google.
So I guess the critic goes to google content filtering algorithms then.
Quality overall dropped somewhat over they years (my subjective impression) but they grew by expanding to many different niches. They also skillfully connected them, so that each one feels like a tight community, yet easy to join.