How do people rate giving a PC plot armor in a campaign?
As I said before, the job of the DM is to cause the characters grief and pain in order to keep them interested in bettering themselves and the world. So plot armor should be applied when keeping the character alive is more interesting to the story or generates more suffering than just outright murdering them.
Killing players keeps everyone ready and on edge, saves them from getting complacent and can make them plan and scheme much harder than normal. You need to let players die when they do something stupid or when they intentionally take the reckless high risk path.
However, there's a lot to be said for creatively sparing people. Changing the last blow to non-lethal damage so that the big-bad knocks the character unconscious and then gates out with them as a captive can create a far greater complication for a party than just executing them on the spot. In one of my campaigns, the ninja trying tail winged-serpent samurai with five levels and a size class over her epically failed their move silently check and ended up trying a direct confrontation. The rest of the party was busy herding the contents of a village into the woods before the could be enslaved or massacred (depending on which clan of the winged-serpent folk got there first) and the result of the battle was kind of a foregone conclusion. In that case, I had the winged-serpent dodge everything the ninja threw at it, give a nice mocking speech about how it didn't even want its servants to have to debase themselves to cleaning the blood of such trash from its blade, and just had him sunder the Ninja's weapons, knock them unconscious with the back of their sword, shatter both their legs, and just leave them to die.
Yes, the ninja should have died. However, by letting them live their character gets a personal grudge and the party gets a better understanding of the kind of enemy they're facing. Not to mention they had to find a way to fix the Ninja's legs since I made the damage too extensive to be fixed by standard healing.
When about to kill a player, ask yourself this question: Is there a way that them surviving would be worse for the party/more interesting for the story? If so, strongly consider nudging things in that direction.