Quora was already absolutely unusable back before GPT-2. It became unusable as soon as people realized that all they had to do was self-identify as an expert to get taken seriously on there, so people started developing whole lifestyles around building up their Quora profiles. From that point on the actually knowledgeable people weren't interested in contributing because there was no way to distinguish themselves from the people who were faking expertise. AI may have been the final nail in the coffin, but Quora was dead long ago.
Stack Overflow managed to avoid that particular hazard by placing less emphasis on real-world identity and expertise, but it also has been in a long-term decline for many other reasons. The fact that they made such a vocal stance against AI and then pivoted so dramatically is just one example of how much they've struggled to find direction lately.
Just a point of clarification, the user moderator base (its power users) took a strong stance against AI, and the company, chasing every possible dollar, overruled them.
Short term profits over user preference is what happened here
Stack Overflow managed to avoid that particular hazard by placing less emphasis on real-world identity and expertise, but it also has been in a long-term decline for many other reasons. The fact that they made such a vocal stance against AI and then pivoted so dramatically is just one example of how much they've struggled to find direction lately.