The Crimson Binome
Hero
I remember it in regards to picking locks, and how the +2 bonus from proficiency was enough to matter, because you would never need a specialist in order to have a chance of succeeding. I thought there was a general statement about everyone having a chance at any task, which would have been applicable to saving throws, but the website is apparently down right now.Bounded Accuracy, as far as I can recall, has always been largely used to describe the relative flattening of the "Bonus To Hit vs AC" curve with scaling damage as PCs level. It also flattened out some of the math for ability checks and saves. However, I've never seen any mention of hard barriers in regard to the numbers, and I don't recall the above (e.g "You'll NEVER automatically fail a Saving Throw because it's too difficult") ever being an expressed goal of the designers.
If you have a source that I may have missed, please share.
In any case, there are rules in the game which are contradictory because of this. You are supposed to make a saving throw whenever you are the subject of certain effects, but you also aren't supposed to roll when the outcome is guaranteed, so it's clear (to me, at least) that they didn't really anticipate the commonality of many high-level characters auto-failing a lot of saves against enemies who can choose to target their weak points. And on a more general note, it's simply poor game design to stage checks where there's no chance of success.