Historically I would see far more em-dashes in capital "L" literature than I would in more casual contexts. LLMs assign more weight to literature than to things like reddit comments or Daily Mail articles.

I think this is most of it. The most obvious sign of AI slop is mismatched style with the medium. People are posting generated text to Reddit which reads like a school essay or linkedin inspirational post. Something no one did before. So even though the style is not unprecedented, it’s taken out of its original context.