More sophisticated clients require we use dashes correctly. I first encountered it pre-pandemic, so in professional contexts it's not a sure-fire signal of LLM use — Should you see em dashes correctly used in the Hacker News comments or Reddit, for that matter, then it's pretty reliable tell... Usually. ;)
I'd like to have the record show that I've been using them since before LLMs :)
Not sure when I started; my guess is that I got into the habit of using them in LaTeX when writing my thesis, and then at some point realized that they are easily reachable on standard macOS keyboard layouts (via "option" + "-").
As I mentioned above, I've had them easily accessible with a keyboard layout for >20 years on all the systems I've used — the only caveat that I find it really ugly with no spaces around em-dashes, which is usually recommended for English.
