http://www.avclub.com/article/these-actors-didnt-survive-their-last-tv-roles-255390

Strangest fact: It took a long time for television to get past the denial stage of grief. Wikipedia’s list is in chronological order, and the most common phrase in the early entries is, “character written out without explanation.” At best, characters like Gladys Kravitz on Bewitched (Alice Pearce died of ovarian cancer during season two), or Mr. Wilson on Dennis The Menace (Joseph Kearns died of a cerebral hemorrhage after roughly 100 episodes) went out of town on vacation and never returned. Often a deceased actor’s character was never mentioned again on screen.

It wasn’t until 1977 that a series publicly acknowledged an actor’s death (at least, as far as this list is concerned), when Chico And The Man star Freddie Prinze committed suicide. Co-star Jack Albertson broke character to thank the audience for its outpouring of remembrance and support, and while the show initially used the old standby of the character going out of town, an episode acknowledged that the character had died. Almost immediately afterwards, shows began addressing actors’ deaths by having the series deal with the character’s death.

No. George Cleveland's death on Lassie preceded it in 1957. Dan Blocker's death on Bonanza was also addressed. Don't know if there were any others. Looking at the list the author of the article obviously missed them because they're listed.

Does Sesame Street count when Mr. Hooper died/when the actor died?

Does Sesame Street count when Mr. Hooper died/when the actor died?

Sure, but that was after Chico And The Man.

Everyone is forgetting about the death of Peter Duel of ALIAS:SMITH AND JONES. But THE SHOW did not ADDRESS his death...Another actor was cast in the role almost immediately!!

Yep on Peter Duel. And on Bewitched the original Mrs Kravitz died and was replaced by Sandra Gould. On Green Acres the original Doris Ziffel died and was replaced by another actress. On Dennis the Menace Mr Wilson died and was replaced by Gale Gordon. That one was explained by Mr Wilson going out of town on a job or something and his brother moved it.On the Andy Griffith show Floyd the Barber died but he was not replaced. They got i n he color eps a new guy who repaired tv's and stuff named Emmitt. I never knew Floyd had died until probably the 80's. But in theses shows they never said anyone had died.

Joan Bradford’s death on “Eight is Enough” was addressed several months before “Chico and the Man” acknowledged Chico’s death.

So when Albertson broke character that was during the live NBC broadcast of that episode at the end during closing credits?