Why kill the em dashes? They are a legitimate mark — one that I have been using for years. On a Mac, you quickly get used to pressing shift-option-hyphen. Em dashes are typographically correct and should be used.
Or at least match the pre-llm human usage frequency.
A total, artificial lack of em dashes can in itself be perceived as another sign of AI writing.
Honestly I don't see the em dash as for sure AI. It's just that it's often used in LLM tropes like the "it's not XXX — it's more" and other tagline-esque sentences.
Because no one knows what those are, and they're a major indicator of AI writing.