They weren't "relatively rarely seen". If you have seen a Word document, chances are good that it had em-dashes in it simply because it would often autocorrect to that. In the Apple ecosystem, this sort of autocorrect is provided by the OS itself, so it extends to a lot more content produced.

I'm pretty sure that all the comments about how it was "rarely seen" are because people weren't paying attention to them before in the way they do now.

In any case, to dismiss something as AI slap based solely on this one thing is both lazy and rude, and should be treated as such.

Its how they're used though, not just that they are used. For example, hyphenating words is common enough, but what wasn't all that common prior to LLM's is people using them to combine sentences -- like this.

In general while reading blogs, Reddit, HN, youtube comments, X posts and so forth, I rarely never saw run-on sentences like that combined with em-dashes. Sure, they existed, but it was pretty uncommon up until about two years ago and now I see them all the time. So anecdotally, there is absolutely a shift in usage right around the time that LLM's took off. I wouldn't judge a professional article or book by its use of em-dashes, but I absolutely judge user generated text on the web.

Its not the only thing I judge on, though. Its just one of a number of red flags. Frequent uses of lists (of usually three items) and of bullet points too -- and run on sentences, not just the em-dashes. Yes, I did that on purpose.