Sometimes they just disappear (https://tvtropes.org/pmwiki/pmwiki.php/Main/ChuckCunninghamSyndrome), and sometimes writers put a little more thought into it.

I guess. I imagine it has more to do with the reason the actor is leaving.

This one's not from a sitcom but it's a good one. On the medical drama E/R the character of Dr. Romano (recurring) and who was quite unlikable, was killed off on the show when a malfunctioning helicopter fell on him and killed him.

Ed.

Charlie on Two and a Half men. At first they said a train hit him, and when he later turned out to be a live, a piano fell from the sky and killed him.

I like the way Edie died on “Desperate Housewives”. I thought it was done very well and unexpected.

Lucy Knight (ER) being stabbed to death by a patient

Dennis the Menace: (1959 TV Series)

George Wilson had to go East. ( The fact is the actor who played Mr. Wilson sadly passed away in the middle of Season 3).

Later Martha Wilson was said that she went East for a while.( The Fact is the actress left the series. She felt her Character should be with her husband George out east). To she was ready to pursue other interests.

The dog Fremont also seemed to be written out when Martha Wilson left the series. (The fact is no explanation was explained.It was assumed that he went East with George & Martha).

when Larry Hagman died of cancer in-2016?-they had J.R. commit suicide by having someone shoot him on TNTs revival of "DALLAS".