Spoilers ahead for the Season 3 finale of Supergirl, called "Battles Lost and Won."
Supergirl made news a few days ahead of its third season finale thanks to the announcement that Jeremy Jordan would be leaving the series as a regular. At the time, details were scarce about how and why Winn would no longer be around full-time, although the executive producers' comments at least indicated that Winn wouldn't be killed off in the finale. He'll be a recurring character in Season 4, and the Season 3 finale revealed what could possibly be enough to motivate Winn to leave his friends and family behind. As it turns out, the reason for his departure also accounts for another character getting upgraded to regular status.
Early in the Season 3 finale, Winn was shocked when Brainiac 5 informed him that the technology he developed at the D.E.O. would go on to save countless lives in the future. Winn wasn't inclined to believe Brainy, as his technology was not enough to save the life of a D.E.O. agent that lost his life in the battle against Reign. Brainy proved his claim by showing Winn a scrap of paper that he pulled from the National Archives in the future. It was a scrap on which Winn had scribbled his ideas for the tech, and his tech apparently goes on to be such a success that the scrap was deemed worthy of preservation in the future. Brainy proposed that Winn take his place among the Legion of Super-Heroes in the future.
Flattered and excited as he was at the reveal that his technology would go on to save countless lives, Winn didn't immediately jump at the opportunity to travel to the future. His home and the people he loves are all based on Earth in 2018, and leaving them would be a very big deal, even despite Brainy's assurances that Winn's activities in the future would be for the greater good. After a chat with James and other friends, Winn realized that he did want to take on this challenge, and he got a D.E.O. going-away party that was a combination of heartwarming and heartbreaking. Winn decided to go into the future, leaving Brainy in his place. Thus Jeremy Jordan becomes a recurring star and Jesse Rath, who plays Brainiac 5, becomes a regular.
Winn didn't go to the future all by himself. Despite his previous intention to stay with Kara, Mon-El learned something troubling about the future and realized that he couldn't just abandon the Legion, no matter how much he loves Kara and is done with his marriage to Imra. He gave Kara a Legion ring, which will undoubtedly come in very handy and provides a way for Kara to call the Legion back to Earth if necessary. Winn (and Mon-El) may be gone for now, but there are ways for them to return in Season 4. We'll have to wait and see. While it will be sad to see Team Supergirl continue without Winn and I'll admit at sniffling a bit at his goodbye with Kara, at least fans can rest easy knowing that he's facing new challenges and fulfilling his potential.