The death of Radar

M.A.S.H. ran from 1972 to 1983. The character of 'Radar' was incredibly popular at the time, and many fans very genuinely upset when they remember him being killed off at the start of season 8. However, they are even more surprised to learn today that he wasn't killed off at all, and went on to appear in later episodes alive and well.

The details are explained at Wikipedia here and the Mandela Effect confusion covered here.

In the current story line, Radar became overworked and stressed after receiving the news of his uncle's death back home, so he took a hardship discharge and effectively resigned.

Gary Burghoff

Radar was played by Gary Burghoff, who was born in 1943 and is also well-known for his portrayal of Charlie Brown in the 1967 musical You're a Good Man, Charlie Brown. His portrayal as Radar in the M.A.S.H. movie of 1970 was his first film role, and when the TV series continued from it, the only actor to continue in the same role. He was nominated for 6 Emmy Awards, and left in 1979 due to "burnout".

He had varied interests after leaving the show. He represented IBM computers and BP gasolene, invented a new toilet seat handle, played jazz drums professionally and was an accomplished wildlife painter.

Good Bye, Radar

There was a two part special aired when Radar left the show, but he didn't die in it.

People become attached to popular TV characters, and when thinking back often cant remember what happened to them. Most people, thinking the dominant theme of the series was war and hence saw meany deaths, would have just assumed Radar was one more casualty and forgotten his return, even when they actually saw it.