 6:12 AM
Personally I never use \\ in (La)TeX's own formula environments, i.e. $…$, $$…$$, and $…$, in MathJax for compatibility with real (La)TeX.
Plus such use in MathJax lacks alignment between lines. 6:40 AM
@EmilJeřábek It is clear that this is not correct in LaTeX. Whether or not this is suppose to work in MathJax is a separated question. There are a few related comments and links in the MathJax chatrom. — Martin Sleziak 17 secs ago
A related post on Mathematics Meta:
10  $P(E) = \dfrac{\text{ 1 choice for C1 } \times ... \times \text{ 1 choice for C(k - 1) } \times \text{ 1 choice for C(k) } \color{red}{\times N} - k \text{ choices for C(N - k) }\times N - k - 1 \text{ choices for C(N - k - 1)} \times ...}{N!}$ How can I force the red onto the next line of the ...

10  \\ works: $$P(E) = \dfrac{\text{ 1 choice for C1 } \times ... \times \text{ 1 choice for C(k - 1) } \times \text{ 1 choice for C(k) }\\\color{red}{\times N} - k \text{ choices for C(N - k) }\times N - k - 1 \text{ choices for C(N - k - 1)} \times ...}{N!}$$ $$P(E) = \dfrac{\text{ 1 choice for C... 4  In addition to Lord_Farin's options, I'll include one-and-a-half using multline (which he apparently does not favour).$$P(E) = \dfrac{\begin{multline}\text{ 1 choice for C1 } \times ... \times \text{ 1 choice for C(k - 1) } \times \text{ 1 choice for C(k) } \\ \color{red}{\times N} - k \text{ c...

I see that there were also some comments from MathJax developers related to this:
Is this just about text mode macros? MathJax only handles math mode, see docs.mathjax.org/en/latest/tex.htmlPeter Krautzberger Dec 12 '15 at 9:14
Peter is correct, MathJax does not process \ outside of math mode, and never has, so you are misremembering it working in the past. — Davide Cervone Dec 12 '15 at 10:47
I certainly agree that it is better to use MathJax in ways compatible with LaTeX - when possible. (Of course, it is not always possible - for example, \newcommand is in the preamble in LaTeX and it is between dollars in MathJax.)
But it seems to me that it worked in MathJax for a long time, so if I had to guess, the intention of MathJax developers was that \\ creates a newline. (But I wasn't able to find something like an "official docummentation" to that effect.)
Davide Cervone mentioned in the GitHub issue.
> Linebreaks are not yet implemented in version 3. This is one of the significant features from version 2 that has not yet been ported to version 3.

1 hour later… 8:00 AM
putting line breaks into non-display maths is certainly not something one does in TeX, why should it work in MathJax? LaTeX has environments to deal with this. 8:48 AM
@DimaPasechnik Well, if not for any other reason, it worked in MathJax for a long time - so removing this would break the display in many places that are using it.
2
@MartinSleziak I see, I wasn’t aware of that. Usually, MathJax strives to be compatible to LaTeX, so this is an odd exception. — Emil Jeřábek 43 mins ago 9:42 AM
This is a very naive search - probably still with many false positives. It returns about 1700 results on MO: link
Even if there are some false positives, it seems that there are users who use \ as newline quite often: link.
I meant \\. 10:38 AM
For comparison, the same query returns more than 25k posts on Mathematics .
And here are the top users.
If I remove/increase the restriction to show top 25k results, the query times out. 10:56 AM
It seems that I have a few posts like that.

5 hours later… 4:12 PM
There is no difference between align and align* on stack editor ?
2 4:30 PM
in TeX, LaTeX and Friends, 1 min ago, by Joseph Wright
@Wolgwang I think for MathJaX there's no real reason to have a difference between the two: you are not likely to be tagging equations

3 hours later… 7:04 PM
@Wolgwang That's correct. There was recently a post of this on meta that can be taken as an example
I currently can't get it explicitly heh, but I could get a piece of it math.meta.stackexchange.com/posts/comments/154015?noredirect=1 you'll see Xander's comment, ignore it, and check the answers from there. 7:25 PM