Let me weigh in by pointing out something that hasn’t been touched on here before.
Bold, italic, underline, and strike have been the basic formatting possibilities since (at least) the BBS.
The spec never mentions the words “italic” or “bold”. These are not actual features of the Markdown syntax. Instead there is “emphasis” and “strong emphasis”. Neither does Standard Markdown specify that these must be styled with italics and bold styles respectively.
Markdown offers no way to “represent text that should be stylistically different from normal text” (maybe it should?) so there is nothing in Markdown that would be complemented by adding an additional syntax to support something as specific as underlining text.
Instead you could set your renderer to mark emphasis by underlining rather than any other styling. In the case of HTML this would be done by setting custom CSS.