Slack and WhatsApp do it, too. I bet also in most of the other major messengers around right now (Signal, Matrix, Telegram). It's a thing in MarkDown, as well. Now that I think about it, some of the only places I type prose where that's not what surrounding underscores do, are HN and various Apple programs (mail, notes, et c.)

[EDIT] For that matter, we all did the same stuff in ascii for years. Asterisks for emphasis (bold, now, in many places), underscores for underlining (which, in the case of, say, book titles, is better represented as italics if you can print italics instead, so I'd guess that's where the "underscore = italic" thing comes from)