I've seen skills on the various skillz marketplaces that specifically instruct the LLM-generated text to replace emdashes with hyphens (or double-hyphens), and never to use the "it's not just <thing>, it's <other thing>" phrasing.
Also to, intentionally introduce random innoccuous punctuation and speling errors.
I do wonder if the way people speak is starting to change because of LLMs. The “it’s not just” thing (I forgot the name for it) is something that used to be a giveaway, but I am now seeing more and more people use it IRL. Perhaps I am just more vigilant towards this specific sentence construction that I notice it more?