In my eyes, I don't really find it contradictory for RPG bosses immunities to be valid across "all bosses" because it's easily explainable by power scaling. If a Rock Golem in the beginning of a game is immune to instant death from a level 10 protagonist, then I feel like they should get immunity to Death spells, but not on the same levels as the Final Bosses, considering the protagonist is 10 times stronger than in the beginning of the game, and the Final Boss is far beyond the first boss. Why not scale resistances in this regard?
Not only that, but on "what we should and shouldn't count as a feat", when it comes to instant death, the only way we can reconcile the idea that a character with ID hasn't OHK'd the bosses is that they can't do it. Otherwise it's either the characters don't have instant death or PIS, and it's subjective to really say if this is PIS or not, since we have a good reason to assume they have instant death resistance in story (they might be too strong to instant kill or have defenses against it because, of course they would), but we also have a meta reason to disregard that (if I could instant kill every boss with Mudo, don't you think the game wouldn't be fun).
In my opinion, I think it's worse to assume PIS than to assume they have an immunity/resistance. Not just because it assumes the writers are incompetent, when really we have proof of a reason as to why they don't get OHK in the form of the game and basic "Oh, he's too powerful!" Logic. And if we deconstructed everything into "gameplay mechanics", which is an arbitrary line, we could destroy every game feat in existence.
If it would appease Rep... Can't we just put a "possibly" in front of the death resistance, and put this at the bottom of the list. As I said, do what is most effective and efficient. If this has to be the last thing ever done, so be it. It just should be done.