Star Trek: Voyager remains one of the few shows in the history of the franchise never to explore the mirror universe. The Mirror Universe, first seen in the original series, is a universe much like the one that Star Trek fans are used to, just with some serious changes. Often times the Federation we all know and love is depicted as tyrannical and oppressive, and characteristics of favorite characters are often flipped, representing a more twisted, or calm version of the characters we know.
The Original Series, Deep Space Nine, Discovery, and Enterprise, with the Next Generation, Voyager and so far Strange New Worlds representing the lone live-action shows that haven't explored the Mirror Universe. Prodigy and Lower Decks haven't either, but as they're animated, the expectations for them to explore it just aren't there as they are with the live-action shows.
So while several shows haven't done the Mirror Universe concept, they did do their own gimmick. The Next Generation dove into alternate history a lot, depicting what could've been, like in the finale of the series. For Voyager, they went a different path with their episode "Living Witness".
The episode starts 700 years into the future and finds two sets of aliens who are in conflict with one another. They learn of the Voyager ship, having once crashed on an alien planet, and its crew. The aliens piece together what they can from the knowledge they have and create a highly dramatized version of them through their findings. After finding a copy of The Doctor's program, The Doctor starts to put things into perspective for them and helps them re-write their history over who the crew of the Voyager was.
The story depicts a different cast of characters that we're used to, like a more unhinged Kathryn Janeway, a more violent Chakotay, and The Doctor, who isn't a hologram but a cyborg. It works for what the writers' are going for, but it doesn't work as an Elseworlds story. The characters we see in this episode aren't real in any context. They're fictionalized versions of the characters that were created by aliens who didn't have enough information on the matter.
It's not like in Enterprise where we had a few dedicated episodes set in the Mirror Universe, where we got to see a different reality. This was just a guess and a wrong one at that. As an episode, it's pretty solid, but as an Elseworld story, not so much.