Plot armor is a term that is grossly overused on the interrawebs, I think. I certainly never encountered it in my studies in English, either at the BA or MA level. It's not generally accepted in any academic journals I've ever read, where we might have prefered the term "contrived" or "facile" in regards to plot devices. I certainly never encountered "plot armor" until about 2008, 2009 or so.
There are times when it's useful as a term, and there are times when I think it has become a cliche of its own, an easy way to score points in an argument and look worldly-wise, cynical, and above it all.
If Alexandre Dumas were alive today, and writing the Three Musketeers, The Man in the Iron Mask, Twenty Years After, and all the rest, I guarantee that someone out there on the internet would set themselves up a platform upon which they would expound upon the plot armor of the four main characters, how in spite of their grinding poverty, arrogance, numerous affairs with married women, dozens of duels, and dozens of battles, and all the espionage work, the most they ever seem to get is a flesh wound here and there. [Spoiler alert for a centuries-old work: iirc, all of them except for Aramis are dead by the end of the last book.]
Modern taste seems to state that "it's only interesting when people die" to paraphrase the Eagles a bit. I've had a very long and on-going conversation with one of my readers in the past year about how I feel that G.R.R. Martin is basically locked in Jacobean revenge drama mode, in which almost all the characters are distasteful, most of them die horribly (in various snuff-esque ways) for the titillation of the audience, rather than for the uplifting of their spirit or the catharsis of their souls (which is the goal of tragedy).
You can read the Three Musketeers as it was intended--a rip-roaring adventure based on a few historical figures and grandly embroidered upon by a team of writers (If I recall correctly, Dumas generally only wrote the beginning and end of each chapter, and designed each with a hook to keep the reader gripped for next week; his team of researchers usually wrote the boring middle bits.). You can read it as a fascinating reflection on 19th century French social concerns as reflected onto the previous centuries, because of the constant focus on money, the poverty of the musketeers, the far more bourgeoisie concerns than you'd have gotten in a drama by Racine or a comedy by Moliere that would have actually been contemporaneous with the Musketeers' own lives. You can read it all sorts of ways. . . but the characters don't die until it's time to die.
And that seems to offend the modern internet audience deeply.
Now, I won't deny that there are times when the term plot armor is useful. When you're watching an incredibly stupid series on TV, in which the main character is doing insanely stupid things. The main character is fundamentally unlikeable, but everyone around them fawns on them, inexplicably. (Buffy, Game of Thrones, and True Blood, I'm looking at you. And don't get me started on Warehouse 13.) They can't die, else the series would be over, and yet they continue to do stupid, inane, mind-bogglingly dumb shit. That is when the term plot-armor is very, very applicable.
I don't think anyone in Edda does insanely mind-bogglingly dumb shit. They do make mistakes, but they make their mistakes derived from the internal workings of their characters. And then they have to pay for it.
In SoR, I could have killed off Rel around chapter 100. I didn't, because it was far more interesting to see him and Dara grow as people by having to learn from their mistakes and fix them. What would that story have looked like if he had died at chapter 100?
No revocation of tal'mae.
No scene with Maxwell completely undoing four thousand years of tradition in a courtroom.
No extended tension between Dara and Eli. No culmination after she nearly dies on Khar'sharn.
No growth from Seheve. She'd have likely died, as slated, taking out the Hegemon.
Kallixta's origin as the bastard daughter of the Imperator and his chief bodyguard would never have developed.
Dara's rachnification probably wouldn't have occurred.
The seminal scene in which Eli puts his marks on Dara in his own blood? Unnecessary.
That's a short list. There are probably other items I don't recall. All that, because I don't waste characters, killing them on a whim. Because complexity is better, in the long run, for themes and growth. Don't get me wrong; I'll kill characters. I kill characters in Edda. But I do it for a reason, and not just to show how serious a writer I am.
In Edda? You're looking at the fight between determinism and free will. Sophia's prophecies suggest that this is a completely deterministic universe. Every time her words come true, and the world moves closer to total annihilation, tension builds, because Sigrun fights prophecy and determinism that much harder.
It would be completely contrived to have the heroes of prophecy, who have been foretold to get all the way to Ragnarok (except the ones who've already been told that they won't), suddenly fall over because of a need to prove that they don't have plot armor.
They do. Sort of. Until they don't.
And that's why I detest this term.
Now, the question is, is this universe really deterministic? Sigrun doesn't think so. Sophia does. And to find out which sister is right, you'll have to read the next two books! :)