Lots of things use underscore for italics, if you asked me to guess I'd certainly try it first (and asterisk for bold).
... in fact I just tried it and underscore apparently works in WhatsApp, TIL.
OTOH /italics/ reads to me as a regular expression, which ... is apparently not what you meant.
(as ever, "intuitive" varies wildly between people, and the trick is to pick the option that's intuitive for the highest possible percentage of your expected user base)
At least in English, typographic conventions have historically equated underlining with italics, and the reason the underscore key is present on your keyboard is because it was used on typewriters to format text that would have been italicized in printed text.
Surrounding text with underscores to indicate italicization is intuitive to anyone who is familiar with that convention.
Personally, I find surrounding text with forward slashes exactly wrong for italicization, because I mentally apply a skew-transform to the text to make the slashes into vertical lines, which leaves the text itself slanted in the wrong direction. Backslashes would make more sense, and also avoid looking like regular expressions. But literally no one uses that convention and we do not need a new one.
I've described what intuition is, you haven't addressed it, familiarity with convention is not intuition, it's just knowledge. And given that convention is unintuitive (_ for underlining would be, and on typewriters underscores were also used to, you know, underscore text), that's a bad convention
Also, this /convention/ already exists and used, despite its low popularity (eg orgmode or Bear note app), so you're also literally wrong here
He said the convention that doesn't exist was -backslashes-.
Also, intuition is "understanding without conscious reasoning," so remembering a convention IMO counts.
Consider e.g. parental intuition of noticing subconsciously it's too quiet and immediately going to find out what the kids have done.
For UI, consider the floppy disk icon often used for 'save' - it's considered intuitive because the vast majority of users already recognise it without having to think about it, even if they've never had a system with a 3.5" drive themselves.
"Intuition is the ability to acquire knowledge, without recourse to conscious reasoning or needing an explanation" per wiki
The convention is the precise rule that _ is italics, so no, you aquire no knowledge, you need to already possess that knowledge and remember that precise rule.
On the other hand, if you know that _ is __underline__, - is --strikethrough--, then you can acquire knowledge (guess) that / is /italics/ because the principle is the same - formatting text follows symbol's shape (though it's not a perfect mach since / transforms text, not adds new symbols unlike _ or - ). But that's still intuition (* for bold would not fit here)
The other bad analogies don't help resolve it, so best stick to the topic
Underscores indicate emphasis, just like underlining would, but in text you usually get italics (I'd note that this is often an <em> tag so it being italics is a browser default style).
Plus underscores get used -lots- of places for italics; I've yet to see the / form in the wild.
A markup language being (as here) used for technical purposes can easily be interpreted according to geek writing conventions.
Did they teach you in high school the relevance of that history? Does the fact that it'd been traditionally used as a substitute mean you can't use underlining now, so it should not event exist in markup?
... in fact I just tried it and underscore apparently works in WhatsApp, TIL.
OTOH /italics/ reads to me as a regular expression, which ... is apparently not what you meant.
(as ever, "intuitive" varies wildly between people, and the trick is to pick the option that's intuitive for the highest possible percentage of your expected user base)