It’s a tangent but two points:

First, the reason LLMs learned to like em dashes is that they are common in the training corpus - they are a thing before LLMs that LLMs have learned, not invented?

Second, work browser has nice blue swiggles under everything I write into a textbox. I dutifully click through them and accept the rephrasing suggestions. I get a lot of em dashes. My blog posts and whitepapers and stuff are full of them and other “AI tells” - but I think they read better because of it.