The best I've got is Stackoverflow survey (are you saying Elixir devs don't use Stackoverflow at all? I find it bizarre. Even if you use Elixir forum a modern dev has questions on many things he needs answered (css, javascript, sql etc).
So please compare https://insights.stackoverflow.com/survey/2019#most-popular-... With https://insights.stackoverflow.com/survey/2020#most-popular-... Elixir was dropped from the list. That's definitely a sign of decline. Now it could be that on absolute terms Elixir is keeping it's usage but only dropping on relative terms but I find it a bit hard to believe. I have a feeling Elixir devs who look for the next gig use other tech more often than not.
The language could be shrinking in comparison to everything an still be growing. I don't think that's the case but it wouldn't change anything for me. I don't work with a proportion of client according to trends. I work with particular companies and I'm at capacity doing Elixir as are many I know.
I believe it has a stronger trend then you believe it does. But neither of us have strong data.
Someone mentioned Elixir has just passed top 50 on TIOBE - that’s growth. It also just crossed top 20 on GitHub by number of pull requests on Q1/2021.
I am not disputing it is niche. I am not disputing the job market is tiny. But for someone who mentions acknowledging the truth, you don’t seem to be very truthful on your claims the language has peaked, especially given the last time you claimed so, you used a milestone (Top 50 on TIOBE) that has been reached since then.