On ther other hand, I would be grateful if you stop questioning the premise for the discussion. Don't call it "ultra-high level". It's dismissive. If not outright false. You know I'm talking about essentially every double-digit level. At least one third of the game.
The focus on level is very relevant form a balance & design perspective. If the problem affects 1/3rd of the game (I would argue it's probably closer 1/4th or 1/5th, but whatever), then the solution should be focused on those levels, and need not focus as much on low levels. If 5% of saving throws are an issue, the solution shouldn't equally or noticeably impact the other 95% of saving throws.
Finally, what do you think about the F/R/W suggestion?
I think it's flawed. And I don't think it fixes the problem.
With two proficient saves, a character is proficient in 2/6ths of saving throws. A character proficient in one of three saves has similar odds of success on any given saving throw. Reducing the number of saves doesn't change that ratio. Yes, it means you have one save you're really good at, one you're so-so, and one you're poor with (rather than one good, two to three so-so, and likely a couple poor). But you still have the saving throw where you have a low bonus.
Similarly, increasing the odds of a character having a good ability score to add to their save doesn't significantly change the chances of succeeding with a non-proficient saving throw, especially at high levels. At high levels you might still have a high disparity between DCs and save bonuses; the change might only be a +1 or +2 bonus at most. So instead of facing the DC 21 saving throw with a +1 they're facing it with a +3. Which isn't all that much better. So it doesn't entirely resolve your problem.
However, at low levels - when things were arguably balanced - you still have that +1 bonus to your worst save, and your middle saves are much higher. So it makes saves easier across the board. Even using your "1/3rd of the game figure" it has an unwanted impact on 2/3rds of the game.
So just in a design vacuum it seems like poor design.
However, games don't work in a vacuum and interact with people. This might leads to the problem that occurred in 4e where characters have three dump stats with minimal game impact. This promotes and rewards min-maxing: you're encouraged to have Con & Dex since those are the stronger of the two paired stats. And once you have one character with a high Cha, the rest of the party is better off with Wis.
This is unless your class hits off a weaker stat. But those classes become slightly inoptimal and more MAD. Classes like the Strength based fighter or the wizard become subtly weaker, since they have a stat overlap.
And it also makes many monsters tougher. For example, dragons. You used to target a dragon's Dexterity saves because they were the weakest. Now, since it's Reflex (and Intelligence) the dragon becomes dodgy. They're the most egregious, but any monster that has one or two dump stats becomes slightly more likely to save. And since - unlike PCs - monsters aren't build via a point buy, they'e more likely to benefit.
Q. Second, what about monsters facing the PCs?
A. What about them? I'm not talking about that.
It's no more fun for the DM to face an impossible saving throw. To have their cool monster killed with no chance to succeed.
And it's less fun for everyone else at the table who might not get a chance to act because their initiative was lower than the spellcaster's. Yeah, most level 15 PCs are just going to fail against the CR 17 goristro's DC 21 Strength saving throw (the lowest CR creature I could find with a DC above 20). But with a -2 it's never going to make a level 17 PC's DC 19 Intelligence saving throw. Ever. And with a +0 Dex, +1 Wis, and +2 Cha it's not going to make many other saves either, even with it's Magic Resistance.
Feeblemind is an auto-win.
Q. the solution to this problem is to reduce PC spellcaster accuracy - which is problematic - or calculate monster saving throws differently than player saving throws
A. Now you have lost me. What problem? I'm not talking about PC spellcasters. I'm not concerned about monster saves. Again, this is not about monsters facing the PCs.
It's the same problem. Both sides face an identical issue: the increasing disparity between save DCs and save bonuses. Because saving throws don't following the standard rules of DCs (aka bounced accuracy).
Which is tied to spellcasters having DCs that increase, impacting their "accuracy" in combat.
That's the root cause of the problem. If you want to fix the problem go after the actual cause. Perhaps change how saves are calculated, possibly just for monsters, or possibly for all characters.
Then: asking me how many times this has actually happened is first of all irrelevant. Secondly, it is insulting, since it insinuates that unless I meet some arbitrary quota of yours, the problem isn't large enough or even real.
There are a lot of "theoretical" problems posted on these boards. Things that are an issue on paper, upon reading the PHB, but may not a serious problem at the table.
I don't think it's unfair to ask if something is a problem that has never occurred but *might*, a problem that has occurred once, or a problem that has occurred repeatedly.