Supernatural ended its run without giving closure to one of Sam Winchester's most important relationships. On many TV shows - as in life itself - death is a one-way street. Characters who die are gone, provided there isn't some creative loophole such as the body having never actually been seen. Conversely, Supernatural spent most of its 15 seasons making it clear that death is far from final within its universe, especially when Sam and Dean travel to Hell or Heaven and back many times.
No matter how sad it was when a particular Supernatural character was killed off, chances are they would still get some kind of post-death appearance later as a way of saying a final goodbye. This was true of both major and minor players and led to lots of tearjerker moments in which Sam and Dean got to say the things they wish they could've said before losing that person the first time. Most notably, the Winchesters got emotional reunions with both of their deceased parents before Supernatural signed off.
Mary Winchester got to reconnect with the sons she never got to watch grow up, and in the 300th episode, John Winchester returned via magical time travel long enough to share some tears and hugs with his wife and kids. Sadly though, Sam never got any closure with one of the people he loved the most, girlfriend Jessica Moore (Adrianne Palicki). That's especially odd considering that Jessica's death was the inciting event that set Supernatural's journey in motion.
Up until Jessica was killed, Sam had no plans to return to the life of a hunter as he'd planned to marry her and settle down. Her death was a major trauma for Sam, yet he never received any real catharsis for her loss. Sam mentioned he still thinks about her during season 15, so the pain never went away. But unlike most everyone else close to him that died, Sam never encounters Jessica after her demise. When Lucifer manipulates Sam into possibly becoming his vessel he uses Jessica's image, and she's also part of an idyllic world a Djinn temporarily traps Dean in, but there was never a real reunion or proper goodbye between the two lovers.
While it's true Sam and Dean did eventually get revenge on Azazel and the lower-level demon Brady that killed Jessica, that's not the same as getting closure. What makes this exclusion more puzzling is that Sam went to Heaven multiple times throughout Supernatural's run, and visited dead people while there, but never thought to say check in on Jessica in any of those visits. Even stranger is that when Rowena's magic allowed Sam to resurrect someone, he chose to bring back Eileen instead of the woman he still apparently pines for on a regular basis. Outside of a possible real-life reluctance by Adrianne Palicki to play Jessica again, the only likely in-universe reasoning for her not returning seems to be that since she never knew about Sam's life as a hunter, bringing her back would require too much adjustment for both her and Sam. Still, that should hardly have been a deal-breaker.