I've been belatedly watching "Star Trek: Lower Decks," and in the episode "I, Excretus," the crew participates in personalized, individual drills. Beckett Mariner enters a holopod and sees that the title of her drill is "Mirror Universe Encounter." She responds: "Mirror Universe? This is easy. Yeah, I can pretend to be evil. Ha ha ha ha!"
Now, the canonicity of Lower Decks is debatable (although it's firmly part of my own headcanon), but it got me wondering how well-known the details of the Mirror Universe are within Starfleet, or more broadly, to the Federation (or other starfaring organizations, I guess) at large. Discounting Lower Decks itself of course, where the answer for Starfleet at least is obviously "Quite well known."
Around eight years ago, I rewatched Deep Space Nine, which featured the largest number of Mirror Universe episodes when it originally aired, and the impression I recall from then is that it was known to Starfleet higher-ups like Sisko, as a serious but low-probability threat to be aware of (perhaps like Q), but not to the rank-and-file. Is that accurate? Is there any more definitive evidence from any other media?