In trying to get an en-dash to appear in comments, I discovered Mathjax is not a viable alternative either. How come I can't use
\textendash in comments? (Also:
\textemdash.) Are there any other commands that don't work either?
The list of MathJax commands is here. It does not include
\textendash (and many other LaTeX commands that are text-oriented as opposed to formula-oriented).
Also, most SE sites, including this one, do not have MathJax enabled at all. See Which Stack Exchange sites use MathJax?
The only way to obtain en-dash in a comment is to paste it in from somewhere else, or to type it directly on your keyboard (if you remember the key combination). In posts you can use HTML
– to get –.