Most TV departures come with a send-off. A character dies, moves away, or gets a final scene with a hug at the airport. Every so often, though, someone just stops showing up, and the story carries on as if nothing happened.
Viewers tend to notice these gaps long after the fact, usually during a rewatch. Three characters in particular have become the standard examples, and each one vanished in a slightly different way.
Chuck Cunningham on Happy Days

Chuck was Richie and Joanie’s older brother in the early days of the sitcom. Gavan O’Herlihy played him in nine episodes of season one, and the role was recast with Randolph Roberts for season two. The show did establish that he was off to college, yet Chuck was never heard from again.
According to Wikipedia, he was mentioned in a few later episodes, then written off completely, and later episodes showed the Cunningham family with only two children, with Richie as the elder. One explanation is that Fonzie began to share the spotlight with Richie starting in the third season, while the sporadic Chuck disappeared altogether. His absence became so famous that the phenomenon is now called Chuck Cunningham syndrome.
Judy Winslow on Family Matters

Judy was the youngest Winslow child, played by Jaimee Foxworth, and she was a regular presence in the early seasons. She was last seen climbing the stairs in season four, never to return. The show continued for another five seasons without bringing her back or giving an on-screen explanation.
Part of the reason was a shift in focus. The original main cast became largely expendable as the show turned into one centered on neighbor Steve Urkel. There were plans for Judy to reappear in the originally planned finale, but the series was cancelled before that could happen. Her fate on screen remains unanswered.
Ben Geller on Friends

Ben is Ross’s son, and he was part of the show’s earlier seasons. Lists of vanished characters point to him as Ross’s son from his first marriage, who left with zero explanation. Considering how central Ross’s family life was to the plot, his absence stands out.
Ben also shows up on WatchMojo’s list of TV characters who vanished without a trace, next to other well known examples. Unlike Chuck or Judy, he wasn’t swapped out for a new storyline in one clean break, so fans tend to remember him fading from the picture rather than leaving in a single episode. No on-screen reason was ever given for it.