[mattbrewsbytes]

Even before GPT it has become less useful over time because you end up with 8 different answers you have to assess if it matches your question/problem. Now just type that in a GPT and get your answer. As long as its not cutting edge stuff just released you're much better off with a GPT query and clarification prompts to narrow down the answers.

I don't know that you can correlate a single tool as being the reason SO is declining. There are also more widespread LSP tools out there over this timeframe. I feel like we may have also seen a plateau of new languages/frameworks in this timeframe. Usually those drive lots of new questions.