I don't track that, but based on last time when I broke the fallback... it is important to have the fallback. There are cases where MathJax can handle input th

I don't track that, but based on last time when I broke the fallback... it is important to have the fallback. There are cases where MathJax can handle input that is technically not valid in modern latex, and the katex devs explicitly decide not to support that input. My users don't care and have lots of content that works in mathjax only, so I have to support it (since Jupyter compatibility is important, and Jupyter uses mathjax).