I've never seen writing created before the advent of LLMs that used emdashes in the same way and with the same frequency that LLMs regularly do. There's probably some out there but it would be a real outlier. LLMs overuse them to an absurd degree, putting them where most writers would put commas, occasionally semi-colons, or nothing at all.

I count 51 em-dashes on the page, which is extreme. They're also used in places where they don't really belong. It's very obviously LLM-generated, at least in part.

That said, it puzzles me why people don't prompt LLMs to change up the writing style a bit and remove some of the tells.