Unlike movies, television shows are constantly evolving. Given that producers need new storylines to be able to continue the show for years, things done in the past may have to be retroactively changed in order to create new storylines going forward.
Throughout many series, showrunners have found it easy to let go of whole characters with no explanation if they feel they no longer serve a purpose story-wise or don't have much for them to do creatively. Producers even have no problem axing actors for the greater good of the show, which has often led to characters disappearing and for some, without even a reason.
Chuck Cunningham - Happy Days
Perhaps the most notable occurrence of a major character disappearing without a trace is that of Richie Cunningham's (Happy Days. Chuck was present in the first season, usually with a basketball, and then appeared briefly in the second season, however, portrayed by a different actor.
Following his second season appearance, not only was Chuck never seen again but his whereabouts are never even mentioned. Going forward, the Cunningham family would mention only having "two children" - Richie and Joanie. Characters get written off shows all the time, but normally the audience is given a reason and even a farewell episode. However, the phenomenon of a character disappearing from existence has since become known as "Chuck Cunningham Syndrome."
Tina Pinciotti - That's 7o's Show
Show writers often create characters such as siblings to use for a storyline but once they're not needed, it becomes too complicated to acknowledge them again so it's better to act as if they never existed (On Seinfeld, both Jerry and George mention having siblings which never come up again). A good example is when Donna briefly had a younger sister, Tina, on That 70's Show.
Tina appears in one episode of That 70s Show but is never seen again. However, in an episode later that season, the characters ponder her whereabouts, but later seasons make it clear that Donna is an only child.
Judy Winslow - Family Matters
Family Matters began as a show about the Winslows, a working-class Black family living in Chicago dealing with real issues. However, after the introduction of the Steve Urkel character, the show morphed to focus on him, and his misadventures, due to his enormous popularity. This meant less time was devoted to other characters like the Winslow's youngest daughter, Judy.
With the focus on Steve, other characters fell by the wayside and Judy disappeared during the show's fourth season, never being mentioned again, which the producers claimed was due to "budget constraints." Had the show been given another season, producers planned to bring her back for Steve and Laura's wedding and explain her absence, but the show was canceled before they could do this.
Tori Scott - Saved By The Bell
During Zack needed another female character to play off, so the character Tori was created. She was a tough, leather jacket-wearing, motorcycle-riding girl, who was the exact opposite of Jessie and Kelly.
She became a love interest for Zack, appearing in all the final season episodes that Jessie and Kelly didn't. However, when the show aired its very last episode, Jessie and Kelly returned but Tori isn't even mentioned. What's also interesting is that Jessie and Kelly are never mentioned in the episodes in which Tori appears. Recently, in an episode of the new Saved By The Bell reboot, Zack mentions Tori to Kelly, who she, of course, doesn't know who he's referring to.
Gwen - The Mindy Project
that didn't survive the show's run. Notably, Anna Camp's Gwen was billed as Mindy's best friend, having graduated from Princeton together and then moving to NYC together as single women.
Gwen has a daughter named Riley and Mindy is even her godmother, however, with all this history, Gwen disappears after the 17th episode of the first season. Despite supposedly being best friends, Gwen is never mentioned again. Initially a main cast member, Camp was demoted to "recurring" after requesting to be let out of her contract in order to pursue other projects but, instead, the character got dropped completely.
Selby - Mad About You
When Mad About You began, the cast featured a single friend named Selby to contrast married Paul. However, Selby only appeared in the first 13 episodes of the show and is never seen again, but his disappearance gets a mention in the season 5 finale.
According to creator Danny Jacobson, the producers felt the actor's performance wasn't up to snuff which angered him and his management, leading to him ultimately being let go. In his absence, the character Ira was created, becoming a main character for the rest of the show's run.
Jonathan Turner - Boy Meets World
Jonathan Turner's exit on Boy Meets World is perhaps one of the most confounding character disappearances because his final appearance left so many unanswered questions. High school teacher Mr. Turner was a regular cast member from seasons 2 to 4, even letting troubled Shawn live with him for a time. However, in a season 4 episode, Mr. Turner gets into a motorcycle accident and his final scene is Shawn speaking to his unresponsive and bandaged head-to-toe body in a hospital room.
Bizarrely, the episode ends without ever giving a clue as to whether Turner pulled through or not, leaving viewers to think he's possibly dead. The actor portraying him, Girl Meets World, alive and well.
Sara Spooner - The King Of Queens
When Carrie Heffernan had a younger, irresponsible sister named Sara whom Spence has a crush on. However, Sara only lasted five episodes and is never mentioned again. Following the first season, flashbacks throughout the show's run depict Carrie as being an only child.
Again, as shows develop, they shed unnecessary aspects and focus on what works about the show. So, like many shows on this list, producers couldn't come up with Arthur.
John Burns - Taxi
The first episode of Taxi actually partly revolves around a character that didn't survive the first season. The audience's introduction to the Sunshine Cab Company is when Alex (Judd Hirsch) brings the character of John (Randall Carver) to the garage in order for him to pay his cab fare, leading to him ultimately getting a job there too.
However, as the season went on, Jim Carrey. So, his legacy on the show wasn't totally forgotten.
Donna Gable - Kevin Can Wait
killing off Donna over the summer break.
While Kevin Can Wait did give a reason for her absence in season 2, viewers felt the show was brushing her death aside, and not making it a big deal felt inappropriate and strange. Ultimately, the show brought back season 1 finale guest star Leah Remini and made her a main character in an attempt to revive some of King of Queens' magic. However, it didn't work and Kevin Can Wait was canceled after season 2.