It's notable that a major part of the drop happened before chatgpt was even released. Lots of good explanations here already for why that might be.
My personal anecdote is that it's almost useless in relation to modern react (ts&functional). New questions aren't being answered and old ones were answered with react& js or even jquery.
The problem is that chatgpt learned in part from stack overflow no? So I understand people likes chatgpt for being more friendly but if people stop using sites like SO then from where are these LLM going to keep learning?