As an avid Elixir user, your comment got me thinking about how and where I look for help. I realized that it wouldn't even really occur to me to use Stack Overflow for answers regarding Elixir, as the other platforms you mentioned tend to be so active and well suited for that purpose.

This isn't to say that I wouldn't use SO for other purposes, as I actively visit the site for JS/PHP content. Anecdotal evidence to support your insight by all means, of course.

Agree with this. I've had a couple of questions on Elixir & while I use SO for looking up answers to other languages, for some reason it never even occurred to me to ask Elixir questions there. Possibly because the folks on Elixir Forum & the Slack channel are incredibly welcoming. They also have areas specifically for those questions you know/feel are very beginner noobish levels.

I haven't used it, but the prospect of having to use a forum rather than Stack Overflow for Elixir help would worry me

Another reason could be that SO itself is trending downward. I know that there are people, like myself, who don't often use it or who just use it as part of a set of google results for the topic they are researching.