alt Hacker NewsHate to argue with people on the internet, but your graph doesn't actually show what you claim. The TeX data was stable until late 2021, whereas the SO decline started in 2017. I also would expect some correlation so that SO was a drag on the TeX site.
I would ascribe that to these communities evolving differently. There is no reason to assume that the popularity of LaTeX tracks the popularity of programming languages. It's a type setting system. And that doesn't even take into account communities that exist parallel to SO/SE. Surely there exist communities today for LaTeX that have been around since before SO began its life.