Many of these are defined in the MathJax texvc extension [1] which exists to help MediaWiki integration.

[1] https://github.com/mathjax/MathJax/blob/master/unpacked/extensions/TeX/mediawiki-texvc.js

@Pkra just two actually, the others are the plain forms of oiint and oiiint.

@TheDJ I see oint, oiint, oiiint also but it definitely sounds like a niche. In any case, MathJax continues to be open for extending that extension to fit the needs of Mathoid /MediaWiki.

If a symbol people need is missing or looks ugly, they will try to rewrite equations to make it work without the symbol or use some workarounds, e.g. in this case probably something like \int_C and then specify what C means. Those line integrals are common in complex analysis (residue theorem) and hence for example also in electrodynamics, I would estimate the number of articles where they could be used to more than 100 on enwiki.