MathJax [1] still recommends this way of using it:

  <script src="https://polyfill.io/v3/polyfill.min.js?features=es6"></script>
  <script id="MathJax-script" async src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"></script>

Therefore, if you ever used MathJax, by possibly copying the above and forgetting, make sure to patch it out.

[1] https://www.mathjax.org/#gettingstarted

EDIT: To clarify, patch out just the polyfill (the first line in the snippet). You can of course keep using MathJax, and the second line alone should be enough. (Though still better host a copy by yourself, just in case).