Do you seriously believe that this data is “the most relevant thing possible” to assert the claims “Elixir is past its peak” and “is all down hill from here”?

I am not refuting the data, I am refuting the flawed conclusions you are drawing from it. You are taking the decline in usage of one service as an overall indicator of the community. I provided a possible explanation. I mentioned the GitHub ranks as counter evidence. But way to go on the ad-hominem.

I've dabbled in Elixir for quite a lot over the past few years, and I have to agree with the general trail of thought you're showing. If I have a question about Elixir, I don't even think about going to StackOverflow for it. I either go ask on ElixirForum, or Google for my problem (which, in most cases, lands me to an ElixirForum thread rather than a StackOverflow one). I think the surge in the use of ElixirForum is inversely correlated with the use of StackOverflow: the more the forum has grown, the less reason there has been to use SO.

I do worry about the impact it has on these "popularity tracking" services though, since the discussion living elsewhere might (falsely?) indicate that the language isn't attracting developers.

Interesting theory. Let's agree that it's inconclusive then!