If I understand {T99369} correctly, the #math extension doesn't support MathJax any longer. (Please correct me if I'm wrong.)
Here are a few things I found by simply running `git grep -i jax`:
There are, however, some traces of MathJax left in the Math extension's code:
[ ] Translatable messages: mw_math_latexml_jax and mw_math_mathjax. They should be simply deleted from en.json,and qqq.json, and the other languages will be auto-updated.
[ ] Perhaps mw_math_latexml should be removed as well.
[ ] README mentions MathJax configuration.
[ ] A reference image for Special:MathStatus. Actually [[ https://en.wikipedia.org/wiki/Special:MathStatus | opening that page in the English Wikipedia ]] says "Test Comparing the generated SVG with the reference failed."
[ ] Tests.
[ ] Remove `$wgUseMathJax = true;` from WMF's [[ https://noc.wikimedia.org/conf/highlight.php?file=CommonSettings.php | CommonSettings.php ]].
If MathJax is completely irrelevant for the Math extension, then I guess that the only results for `git grep -i jax` should be in the HISTORY file.