The idea of plot armor is an interesting one, and I think I can give a rule of thumb for good versus bad plot armor.
Plots of stories generally follow an understood arc of overcoming adversity. Subversion of that theme can be interesting, but it rarely is as satisfying as actually overcoming the problem at hand. The difference between good and bad plot armor revolves around how the conflict is resolved. If the character doesn't actually overcome the adversity themselves, but instead 'something or someone unexpected' overcomes it on their behalf, it is unsatisfying.
Examples:
Good - A man gets beaten up and bullied. He trains and trains until he's able to beat his bullies. (The man worked hard to overcome his adversity).
Bad - A man gets beaten up and bullied. He suddenly unlocks powers that let him beat his bullies. (The man hasn't changed in any way, his problem is just gone now.)
Good - The hero's party has fallen, and it's just him versus the demon king left. With his last ounce of power, he throws his sword, defeating the demon king. He awakens later, the survivor of the battle, but his comrades have perished, a pyrrhic victory.
Bad - The hero's party has fallen, and it's just him versus the demon king left. Through a power he'd never shown before, he revives his allies and they easily defeat the demon king as they're now immortal.
Good plot armor doesn't mean that the protagonist *could* lose, just that the protagonist has *earned* their victory. It's mostly about the why, not the how. People don't throw around the term Plot Armor in reference to the fact you expect the protagonist to win (it's just expected), even though they also have plot armor. They use the term when the plot itself is forming unbreakable armor around them, and reality itself conspires to make them win, even though they haven't earned it. In all cases of fiction, technically speaking, the author has written to make the hero succeed, so all of them have plot armor, though in some cases, the author has acted as Merlin for the wrong individual.
It could actually be an interesting exercise to write a story where the MC doesn't have plot armor. They die when they should, then the story jumps to a different MC... kinda like Piers Anthony's Xanth series, but switching MCs mid book, rather than between books.
I have an idea for a story like that which I'll almost certainly never get around to writing. I wanted to also change the story genre for each new MC.