would be great if editorial supported underline
Which dialect of Markdown is that from? I don’t believe “standard” Markdown has that in. What I use (and in my code process, but that’s a different story) using the <ins> HTML tag. (You can embed HTML in Markdown - for most Markdown processors.)
Normal Markdown doesn't have any native syntax for underlining -
__underline__produces underline, i. e. bold text. In standard Markdown,
__bold__is equivalent to
_italic_is the same as
Chat applications like Discord, Slack and Mattermost support syntax that is similar to normal Markdown, but isn't completely compatible (they include some custom features that don't exist in Markdown, and don't understand all standard Markdown syntax). Discord's underline feature is an example of that.
Standards are meant to be broken, software darwinisim in action. :-)