I was thinking about this with the last couple episodes of "NCIS" where Ziva was killed in an explosion off-screen. Another one that came to mind was "Criminal Minds" where Gideon was murdered with a stand-in serving as the body (under a sheet).

What other instances has this occurred?

Julie Finlay on CSI

Her death was mentioned in passing on the 2 hour finale

CD was murdered off screen in Walker Texas Ranger.

Danny Sorensen was also murdered off screen on NYPD Blue when Rick Schroeder left the show.

Nicole Chapman on Fame

Henry Blake - M*A*S*H

Going back even further Jim Davis (Jock Ewing) was killed off on Dallas after he died in real life.

Lynne Thigpen died of a brain hemorrhage while working on The District. They didn't kill her off, but made her character die of the same fate. I remember they shut down filming for a couple of months after it happened.

On Army Wives, Kim Delaney's character Claudia Joy died offscreen when she wasn't asked back to the show. It was following her alcohol issues.

While not technically "killed", Colin Baker's Doctor on Doctor Who should count. I think they just had Sylvester McCoy wear a wig to represent Colin's Doctor before the regeneration started.

Most famous, by far, is Charlie Sheen on Two and a Half Men. Twice, actually.

That was off-screen, but it was on the same episode he said goodbye and was done with his consent.

James Evans - Good Times

Are we counting when the actor dies in real life? Because I imagine there'd be a ton of those...

Almost forgot, Larry Hagman (JR Ewing) was murdered off camera on the TNT reboot of Dallas after he died of cancer in real life.

When the actress playing Livia died inbetween seasons of The Sopranos, they had her character die off-screen, but not before using her in one last scene by using old footage to CGI her head on another actresses body. Even 15 years ago this looked terrible.

Many examples already, but it falls within the parameters.

No, it wasn't. Only a very few people knew they were going to kill off Henry, McLean was not one of them.

Private Practice:

That was the first one I thought of.

There was sort of a behind-the-scenes war that ended up with Colin Baker being fired between seasons. He was (rightly) pissed off and refused to return for a regeneration scene.

Sgt. Phil Esterhaus on Hill Street Blues, and Coach on Cheers, due to the deaths of the actors portraying them. I never watched Newsradio. How did they deal with the death of Phil Hartman?

Phil was murdered during summer hiatus. The first episode of the fifth had the characters coming back from his funeral. They even brought back Khandi Alexander for that show.

I seem to recall one of the DA's from Law & Order just disappeared out of the blue. They said her character had been kidnapped and beaten to death.

https://en.wikipedia.org/wiki/Alexandra_Borgia

Howard's mom on Big Bang Theory...although, she was never really "on" screen except for a computer generated shot from a satellite.

you get a few glimpses in this scene, too:

Peter Gregory on Silicon Valley. Christopher Evan Welch died during production of the 5th episode and was written off as being away on some business trip. The second season started with them relaying that he was killed while on safari.

Back in the '70s, Susan Saint James left "McMillan and Wife", which simply became "McMillan". At one point during the first episode, McMillan's sidekick (John Schuck) drops a line (I'm paraphrasing) commenting on how well McMillan (Rock Hudson) is doing since Mrs. McMillan died in that plane crash six months ago.

Biggest one I can think of was Valerie Harper from her self-titled sitcom Valerie. She demanded more money once the show became a hit, contract talks fell through, and she walked off the show. Rather than recast her character (also named Valerie) they simply killed her character off and resumed the show as Valerie's Family, and later The Hogan Family.

I seem to recall the reason given in the show was that a checkout aisle candy stand fell on her and killed her.

Pretty tacky way to treat her, regardless of behind the scenes drama.