It always bugs me that _underscore_ is the syntax for italics rather than underline.
I’m sure back in the old days of READMEs, long before markdown was a thing, the conventions were this:
/italics/
_underline_
*bold*RFC 1855, Netiquette Guidelines[1], specifies underscore for underlining. However, it says asterisks are for emphasis, not bold, per se. They just happened to (often?) display as bold because italics in terminals weren't a common thing. For the same reason, using /'s for italics didn't make much sense except maybe in word processors. I also suspect underscore become conflated with asterisk because some people preferred using the former for emphasis--people weren't usually trying to adhere to professional styling guides, and some people may have preferred underlining to impart emphasis, or just got into the habit without thinking about it.
I don't know how well RFC 1855 reflected common practice, though. It might be worthwhile to check the rendering code in clients like tin and mutt.
Since this seems to boil down to personal choice, has anyone considered a customisable alternative? Like a frontmatter that declares which character is bold, which is italic. You could easily convert between them according to local preference, much like tabs/spaces.
My /usr/bin/ folder wants to know which bit gets italicised :)
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Org mode uses / as well.