Here's an article in the Smithsonian magazine from 1995 with an em-dash:

https://www.smithsonianmag.com/arts-culture/review-of-the-pr...

Where do you think LLM's learned these things from? They are widely used in literary writing. Like magazines and books.

If you read a lot of books, particularly older ones, you'll find em dashes in all kinds of writing and used often. It's functional punctuation that once you understand you may even find yourself using it (and then being accused of being an AI, lol)

Not every single emdash ever used is AI