This is a large decline. It does not correlate well with the rise of LLMs, which have shifted my own habits away from Stack Overflow recently. Looking through the graphs, the most dramatic shift was the large drop in new visits in the first half of last year, by two thirds or more. That seems late to be a Covid effect. There were some political controversies, but it would surprise me if they had such a large effect.…
IMHO people have tired of the smug dismissive assholes that dominate SO and its its insipid gamification and they have, gradually, found alternatives.
For some, it's more welcoming and knowledgeable communities in github issues+discussion.
For others it's special-purpose, interactive forums that provide more guidance and non-hostile support for users of certain platforms/tools (eg Posit Community).
For many, copilot has been fulfilling that need, Does it always give "the correct" answer? No. Does it provide a sketch of a solution that gets you half-way there when dealing with tedious humdrum stuff that you just forgot because it's so boring? YES.
For yet others it's just plain old reddit where you can ask a question and whether it gets smacked down or not depends on how cool the community happens to be.