Honestly, I assumed she died of old age.
No, she didn't!
Loved Kes in S1-3, loved her departure in "The Gift," evolving into an energy blob and sending Voyager 1/7 of the way home. "Fury" was fun, but that should have been a two-parter to give the store more time to work. Wasn't there talk of her being a recurring villain across S6? Anyway, at the end of "Fury," old Kes flies back home to Ocampa. At this point, she's what, 6 or 7 years old? She hasn't acquired the longevity that Suspiria's Ocpampa have benefited from. What do my fellow VOY fans think happened to Kes? Too bad Jennifer Lein can't reprise the role given what she's been through in life.

If she were just retired from acting, maybe they could have cut a big check for a special guest appearance in one of the streaming shows?
What happened was that Kes was about to be turned into an energy being but was rescued by Q who saw her as a perfect candidate for becoming a Q and go to the Q Continuum.
However, Kes didn't want to do that. instead she managed to persuade Q to give her a human lifespan but keep the limited psychic abilities she showed up in episodes like
Persistence Of Vision and
Cold Fire.
Q accepted her wishes because it would help him to solve some problem with too many ships from the Alpha Quadrant causeing problems in the Delta Quadrant.
Then Kes was sent to a planet which was involved in a conflict with an expanding aggressive empire led by a ruthless dictator. Shortly after that Kes was abducted by pirates but was saved by members of a Maquis ship which also had been abducted by The Caretaker some time before Voyager's arrival. The crew onhe maquis ship was currently helping the friendly people on the planet to defend themselves against the empire they were fighting.
Later on, Kes and her new friends encountered Voyager and with a little help from Janeway and her crew they managed to stop the invasion which the dictator of the empire planned.
After some hesitation, Kes decided to stay on the Maquis ship where the crew had decided to stay a while longer at the planet to be sure that the hostile empire wasn't about to plan anew attack. The plan was to meet up with Voyager later on but that didn't happen. Instead, they managed to find a wormhole which led to the gamma Quadrant and from there, they could reach the Alpha Quadrant.
Kes and the members of the Maquis ship settled down on a planet between Bajoran space and the Tzenkheti Alliance where she lives a quiet life, sometimes interrupted with exploring space together with her new friends and exciting adventures. From time to time she run into her friends on the starship Voyager as well.
As for the horrible monster which showed up in an episode which never should have been made, that evil creature wasn't Kes.
It might have been Suspiria or some evil being from another dimension.
OK, that's the
Startrek Universe explanation, here we have the
Gray Universe explanation:
Kes was dumped from Voyager and the actress who played her, Jennifer Lien was bruatlly fired because the narrow-minded fools in charge of the show wanted a "sexy babe" in the show to boost the ratings.
The original plan was to dump Harry Kim and fire actor Garret Wang but when a silly inside poll among the staff of a glossy magazine placed Garret Wang among "the 50 most handsome", those in charge of the show changed their plans, fired Jennifer Lien and dumped Kes instead.
The episode in which Kes "left" called
The Gift was a worthless piece of crap with a thin, unconvincing story which didn't make any sense at all. It looked as if the so-called "writer" had written the script for it while sitting on the toilet, using the paper to write on or during a five minute coffee paus.
Some seasons later, the arrogant people in charge of the show became annoyed by the fans who wanted Kes back and hadf staged a letter campaign for it. So they decided to give them a lesson.
What happened then was that they decided to totally destroy the character in a horrible, sadistic episode where Kes was portrayed as an insane monster who wanted to destroy the ship and kill her friends.
The plans were to kill off the character but jennifer Lien, the fired actress who all of a sudden was welcome to partictipate in that horrible destruction of her character made them change their mind about that so instead the character Kes was turned into a pathetic and crazy old wreck determined to ttavel back to Ocampa in a shuttle with no chance at all to manage with that.
By coming up with that insulting piece of s**t, the worst episode ever made in any TV series, those in charge managed to show the "finger" to those ungrateful fans who wanted Kes back, sort of:
"So you wanted your little favorite back? Well, eat this you scum! And don't you ever question our divine decisions because this is our show and we do what we want. Your only obligation is to sit in front of your TVs and say "yes" to everything we do and decide while filling our wallets by loyally watching our show".
The explanations for dumping Kes were also pathetic lies which have been changed from time to time.
Firts they told us that "Jennifer Lien had left of her own free will" which obviously wasn't too convincing.
So they changed their stiory to "We couldn't come up with good stories for the character" which was an even more ridiculous way to cover up their ghastly business. Can anyone believe that professional writers all of a sudden lose their abilities to write good stories for a character when they had managed to do that up to the last episodes in which Kes was present (with the exception for the episode which was written during a short visit to a toilet)?
Not to mention that there were talks about "mutual agreement" when Jennifer Lien was dumped.
Now, anyone who have the slightest knowledge about movies, TV series, music and showbiz in common knows that everytime the words "mutual agreement" are used, then foul play is involved.
So in later years they had to change the story again, now using actress Jennifer Lien's problems in recent years as some excues to fire the actress all those years ago.
Well, you know the old saying. "fool me once, shame on you, fool me twice, shame on me" so what about "fool me three times........."?
As for a return of Kes in later series, I must admit that I wished for many years that she would return in some series or books.
But considering how bad and destructive the NuTrek movies and recent series like the crappy DSC and the downright lousy PIC have been and how the writers of the Star Trek books are doing their best to ruin, destroy and execute good characters from TOS, TNG, DS9 and VOY, then I'm happy that Kes never have been brougt back to any gloomy 2020's series, book or movie.
Not to mention that it will be very hard to bring back the character without Jennifer Lien's participation. Back in the days when there still were some good acting here and there, I used to think of skilled actresses like Kirsten Dunst, Renee O'Connor, Laura Harris or even Nicole DeBoer as possible replacements if Jennifer Lien couldn't or wouldn't participate.
But as it is today, I have no intention at all to watch some third rate actress, some Melissa Spicegirl with big artificial lips ruin the character further than already have been done.
Due to the doom-and-gloom trend in the TV-series today, they would probably destoy aand ruin he character even more, most likely kill her off.
Thanks for your attention and have a nice day!