[kristjansson]

No one is asking that we reject all prose with emdash. Not all emdash-users are LLMs, but many LLMs are profligate emdash-users, so adjust your priors accordingly.

Secondarily, I think there's a part of the discourse missing: the presence of a syntactic emdash in a sentence on the internet is not itself a strong signal of LLM-writing - but the presence of an actual emdash glyph (—) should raise some eyebrows, esp. in fora that aren't commonly authored in rich text editors (here, twitter, ...)

[genthree]

Before LLMs, the em-dash glyph was a decent tell simply that... the author was using a Mac, because it's a simple and easy-to-remember (or even guess!) key-combo on there. Not that you can't type it on other keyboards, but the Mac one for whatever reason had a combo of users-who-wanted-to-type-it and layout-that-makes-it-easy that resulted in a high proportion of correct em-dash employers being Mac users.

(option-underscore, or option-shift-dash if you prefer to think of it that way)

On iOS, you can type it by simply holding down on the "dash" button then selecting the em-dash from the list of options it presents. It may also correct double-dash to em-dash a lot of the time, not sure.

I have used the correct em-dash everywhere I can for over a decade, which amounts to nearly everywhere.