ChatGPT for example almost always uses them. I'm sure they are more common in academic writing, but its now super common on boards like Reddit.
When looking at the context of a given text, use of certain words or punctuation, can very well indicate AI use.
The "original" example was delve. There is no doubt that AI (did, or still does) use this word at a significantly higher frequency than the average person. I would say the same about em dashes.
When browsing a Reddit thread about a video game, if you encounter numerous comments written perfectly, especially those containing indicators like em dashes, the word delve, or similar language, it certainly can raise the question: am I genuinely seeing comments from users who write this way in this specific context, or is this content more likely produced by an LLM?