I don't think the character is that uncommon in the output of slightly-sophisticated writers and is not hard to generate (e.g., on macOS pressing option-shift-minus generates an em-dash).

In fact, on macOS and iOS simply typing two dashes (--) gets autocorrected to an em dash. I used it heavily, which was a bit sloppy since it doesn't also insert the customary hair spaces around the em dash.

Incidentally, I turned this autocorrection off when people started associating em dashes with AI writing. I now leave them manual double dashes--even less correct than before, but at least people are more likely to read my writing.

That's an en dash, not an em dash. An em dash is longer and as far as I know Libreoffice doesn't have a built-in way to make one (though you may have added it to the autocorrect settings yourself).

en dash: https://www.compart.com/en/unicode/U+2013

em dash: https://www.compart.com/en/unicode/U+2014

Edit: Ah, Libreoffice does have a built-in autocorrect for em dash, but you have to type this:

  :---:

Thanks! I did confuse the two despite knowing of both.

So ":---:" does work for the em dash? I thought something with fewer keystrokes work, too, at least I remember the em dash from less, but perhaps I just typed it so quickly I did not realize it was indeed ":---:".