LLMs are not 'ruining' em dashes. It's just a convenient device to unmask people who make critical judgements based on ridiculous and flimsy evidence.

It is good they are being unmasked. You must avoid those people and warn your children about them. They are not safe to be around.

People always crawl out of the wood-work swearing blind they've always used em-dash, but the truth is that actual em-dash usage has exploded by a factor of thousands and is therefore, along with other markers, a strong indicator.

It's not proof, it is evidence.

"Ruining" in the sense that "I worry that when I [use em dashes] people will assume my writing is LLM output".

I'd feel the same if I was someone who naturally frequently used phrases like "you're absolutely right", or for a much more extreme analogy: if I was a Hindu living in Europe in the 1920s and then the Nazis came along and "ruined" the swastika for me.

I'm hoping it's not the semi-colon; I use that a lot.