Board Thread:Wiki Discussion/@comment-27713488-20180106040608/@comment-31763506-20180112185035

Presumably em-dash is being discouraged for the same reason as the inequalities, because it will not necessarily be consistent when copied and pasted to text editors. It might get transformed  into a hyphen or a double hyphen, and the latter is a bit unsightly. That's a bit less of a problem than &ge; and &le; getting zapped into in = unintentionally, because that actually changes the meaning.

I actually don't see why the &amp;mdash; HTML entity can't be allowed.