The things seem to be back to normal.

This question mathoverflow.net/questions/353758/… uses red letters (to emphasize some constant in a math expression). Since in almost 10 years here I had never seen this, I'm curious whether it's a new feature, whether it has ever been discussed, whether it should be avoided, etc.

I don't know about some way to search specifically for some string (in this case color) inside MathJax. Perhaps when SEDE is again up, it could be possible to search for some regex in the body.

However, you can find posts which use a specific color by the usual search - for example, searching for "color{red}".

I do not see anything on MathOverflow Meta, with exception of one brief mention in this answer: What is the position on titles using \displaystyle, \dfrac, etc.?

On Mathematics Meta that are some discussions, for example: On the use of \color in equations and some of the posts linked there.

Regarding the question whether it's a new feature - clearly not; the above search returns also some rather old posts.

BTW red color is also used in the only answer to that question (current revisions) - most likely the answerer was inspired to do that by the use of color in the question.