Unfortunately a lot of contests etc. are anti-AI usage without having a formal system for detecting it. In practice that means anyone using a lot of em-dashes will be flagged by a reviewer as AI-likely.
Then these contests are run by lazy people and they aren't worth the effort of submitting your work to.
I would say that using a lot of em-dashes was always bad writing. You want to use them sparingly if you want them to have impact.
That said, yes, keep using them (and using them well!).