AFAIK most computer keyboards don't have em dashes. Rather than hit ALT+0151 every time, I've always just strung along two hyphens, like: -- Absolutely proper and correct use of em dashes, en dashes, and hyphens is, to me, the most obvious tell of the LLM writer. In fact, I think that you can use it to date internet writing in general. For it seems to me that real em dashes were uncommon pre-2022.
I always use an em dash when possible when I should, and double en dash when I can't, just because I'm that kind of nerd. But it is the case that a double en dash on iOS autocorrects to an em dash, so I'm suspicious of the claim that em dashes are a tell for LLM writing.
Most editors should auto changes a double dash into em dash. I thing Google Docs does for example.