Ahrimon
Bourbon and Dice
Very dramatic. However...
You can do much less and invalidate the challenge of CR 20 monsters and thus break the system.
It's not the only way to get high AC. There are several ways to get to this level of AC.
This problem will get worse. We don't even have the full list of magic items from the DMG yet with Ioun Stones, Bracers of Defense and the like or the first splat book with new classes, spells, feats and even more magic items.
PCs can reach ACs that encounters rated as deadly can't hit and simultaneously have AB that mean they rarely miss.
As I've said before, there are games out there with bounded accuracy. It's very easy to see the difference between their mechanics and 5Es and there is a difference because 5E is not a bounded accuracy game. It is a game that is intended to be as simple as possible for kick-down-the-door gaming while maintaining some of the feel of previous editions and to be that simple things got sacrificed and one of them was bounded accuracy.
The point that you are missing is that the default for bounded accuracy is NO magic items. None. Zero. Zilch. Nadda. This has been stated by the developers on several occasions.
If a DM chooses to give out magic items then that DM is choosing to stretch or break the bounded accuracy. I would be greatly surprised if the DMG doesn't explain this outright.