[palmotea]

> But the em-dash is a pretty informal mark...

I think you have to make a distinction: there's using a dash as you describe and using the actual em-dash character. Without an smartquotes-type autocorrect-type feature (which admittedly is common in certain apps/platforms like Outlook and Word), an actual em-dash is awkward to type. I'd expect someone using it informally to just use a regular dash (-) or two (--).

I think you're automatically in a pretty formal writing context if you care if you use an em-dash character or not.

Which brings up an interesting idea: would Microsoft turn off it's smartquotes-type autocorrect, because now it makes you look like a dumb AI-user? Probably, if they cared about their users. But I doubt they will because they're so into hyping AI that "Microslop" is a thing.

[lamasery]

It's easy to type, and even easy to discover, on the default Mac keyboard layout. Until recently, the main thing employment of the actual M-dash in web posts indicated was that the user was more likely than not typing their posts on a Mac—not for-sure, but better than even odds, despite Macs having a much smaller share than half the market.