Spoilers for Season 10 and earlier seasons
So we know Supernatural has no qualms against killing our favorite characters. Bobby, Benny, Balthazar, Gabriel (though he might have been revived). At least Castiel and Garth are still alive, not so sure about Crowley, but how long are they going to last?
But they killed probably the coolest character in the whole series at the end of Season 10. We hardly ever see him too so it's a huge treat whenever he appears, and now they had Dean kill him off. Why'd they have to kill Death? (and I'm not talking in-universe, they could have just avoided that whole scenario altogether and had him appear another way)
So Supernatural is joining the new trend for not killing off main villains (Abaddon was the last main villain to die, as Metatron and Rowena survive), but the only reason series are doing that now is because fans like the villains so much they're disappointed to see them killed off, and Supernatural is not worried about that because they continuously kill off our favorite characters. Bobby, Benny, Balthazar, and now Death. I imagine Ellen, Jo, Ash, Rufus, and Meg have something of a fanbase as well.
Again, it's amazing they've kept Castiel and Crowley around for so long, even reviving Castiel twice, and seemingly killing him off at least twice that I can remember, and Crowley once when he faked his death. Crowley is one of the only two main villains to survive before Metatron, other being Lucifer who is imprisoned in hell seemingly with no way out, maybe they just decided to always keep Castiel and Crowley around as the two fan-favorites who never permanently get killed off.
I have heard speculation that Death is still alive and faked his death by giving Dean a fake scythe (if Crowley can pretend to go up in flames, Death can certainly pretend to crumble to dust), except they said a fan-favorite would be permanently killed this episode and it obviously wasn't Crowley since Season 11 reveals he survived.
edit: So his death is 100% confirmed. I've tolerated the deaths of Balthazar, Bobby, Benny, Meg, and Charlie, and the betrayal of Ruby that made her death satisfying, but this is just going too far. The way they killed Death was clearly just a cheap way of getting rid of another of our favorite characters. Like, why would Death give Dean the only weapon that could kill him and tell him to kill Sam with it? The Death we knew would have either given him a different weapon or done it himself. And if they really wanted to kill Death, then maybe it would be acceptable if they had had Rowena release the Darkness first and then the Darkness kills Death to show how powerful she is