Deaths of major characters on sitcoms are fairly rare but obviously they do happen.
Off the top of my head, there’s Edith from “All In The Family”, Colonel Blake from MAS*H and James Evans (the dad) on “Good Times”. John Ritter of “8 Simple Rules” wouldn’t count because he passed away IRL.
Who else is there?
Charlie Sheen as…whatever his asshole character was called on 2 and a half men. I never watched it, but I know they fired Sheen and killed off his character because Sheen was such a prick.
Maude Flanders
Chuckles the Clown
The Hogan Family, originally titled Valerie. It starred Valerie Harper, who was written out of the show in a contract dispute. Her character was killed off and she was replaced with Sandy Duncan.
Depending on your definition of “major”:
Eddie LeBec (Jay Thomas) on Cheers was killed off because the actor made nasty remarks about Rhea Perlman
Chef (Issac Hayes) on South Park was killed off because he left show, objecting to the episode they did on Scientology.
Susan Ross (Heidi Swedberg) on Seinfeld because, well, Seinfeld.
Elaine Lefkowitz on Soap, which was not a traditional sitcom and pretty dark.
Have any come back after being killed off? In fiction the classic example of someone who was apparently killed off is Sherlock Holmes.
Shortly after him (Holmes returned in The Empty House in 1903) H. Rider Haggard had She Who Must Be Obeyed return in Ayesha in 1904-5. The difference is that Ayesha was “undeniably and reliably dead”, unlike Holmes, who was merely presumed dead. Hers is the first case I know of where a certifiably deceased character returned from the dead to continue the series. It wouldn’t be the last.
It’s a soap opera trope. Larry Hagman in Dallas, not officially a sitcom.
There was Cheers where we hoped Diane was dead but turns out she wasn’t.
Nitpick: Patrick Duffy was the one in the shower. Hagman’s character was shot, but not fatally.
Similarly, Charlie Sheen’s character in Two And A Half Men was believed to have died in Paris, but in the final episode we learned he was being held captive by Rose.
Her final fate is unknown, but Stephanie Barnett, the hot MD on Big Bang Theory, just disappeared after season two was over.
Dr. Kudner (Kal Penn’s character) on House; Penn had just accepted his job with the Obama administration.
Ed Marinaro’s character (whose name escapes me) on Hill Street Blues.
Dr. Armstrong on St. Elsewhere - this may be stretching the definition of “major” somewhat, but the circumstances that led to the character’s death were controversial; they wrote the character off of the show because somebody (either the producers or NBC) felt that too many viewers were turned off by the fact that she was Asian.
Brian on Family Guy.
The entire cast of Blackadder II died at the end of the second series but were back as their descendants or isotopes of their old selves for Blackadder III. I suppose you could say the same for the end of Blackadder IV and the various specials since, too.
None of those are sitcoms.
DESPERATE HOUSEWIVES picked up Emmys and Golden Globes in the ‘comedy’ category, and Steven Culp’s character got killed off – dying slowly enough to write a note forgiving his wife for poisoning him, despite the fact that, no, she didn’t.
Coffey. Ed Coffey, I think.
One of the truly shocking character deaths in TV drama. Done more or less in the episode tag, after the main stories.
“Make Room for Daddy” they killed off Danny Williams’s wife Margaret after three seasons because Jean Hagen and Danny Thomas didn’t get along (she felt under utilized). Had her death happened between seasons so season four Danny was a bachelor, single father of two until he met his second wife late in the year.