I think em-dashes were once a reliable indicator (though never proof), but recent models have been fine-tuned to use them much less. Lots of recent AI-generated writing I've seen doesn't have em-dashes. Meanwhile, I've heard many people say that they naturally use em-dashes, and were already and/or are afraid of being accused of AI; so ironically this rumor may be causing people to use their own voice less.

Before, I naturally used hyphens as if they were em-dashes. The kerfuffle over LLM use of em-dashes motivated me to figure out how to type them properly (and configure my system to make that easier). Now I even go over old writing to fix the hyphens.