Why are you sidestepping the basic fact that if a player can trivially/cheaply attain the "cannot be hit" ability the game works (much) less well.
Why contort yourself around this issue when the simple (and correct) solution is to make it substantially more costly to attain this ability?
GMs should not have to work around a character that can only be hit on a 20. Characters should not be given this ability in the first place.
I'm not because they aren't?
They aren't obtaining it "trivially" or "cheaply", generally these are builds, and they are functioning mostly as expected. Shield+Mage Armor is not an issue, unless it is on a Bladesinger who then activates bladesong. And, using three daily resources is rather significant.
But, let's work this out. Either the Bladesinger or the Eldritch Knight. Both can't happen until level 3, both are going to be able to bump their AC to 24. And yeah, at level three? That's crazy. But for level 3 characters, an Ogre with a +6 to hit is a fair fight, and he can hit them on an 18 thru 20. Also, the Bladesinger can only use Bladesong twice per day, and the Eldritch Knight can only use Shield twice per day.
So we are talking two rounds of combat, max, at level 3 for the Knight, and maybe three or four for the wizard. Which seems fine to me? It is costing all of the Knight's spell slots, and doing this once is costing the wizard a total of two spells and half their subclass resource.
Bump up to level 6. Let's say the knight has platemail now, so they can push their AC to 26, and the Wizard has more spells, more uses, and can reach an AC of 25. At this level a Venom Troll isn't a bad match up. With a +7 to hit it can hit the wizard on an 18 to 20, and the fighter on a 19 to 20. IT also auto-damages anyone within 5 ft of it when it takes damage, and can cause a con save versus large damage and the poison condition. So, even if the troll can't hit the party in melee... it is still going to potentially kill either of these characters from its other abilities.
And to be clear, I'm not speaking from ignorance here. I played with a guy who multi-classed bladesinger and got a staff of shielding to get mutliple free casts of the Shield spell. He was nearly untouchable... but the DM also never bothered to send anything against us that wasn't a very simple "roll to hit" enemy, and even then, the one time he did get hit... nearly dropped his character because he didn't have the HP to back up his build.
So, I legitimately don't see the issue. Tank characters with high AC are hard to hit with melee attacks. That sounds like things working as intended. It costs them some fairly significant resources early on, and later on the enemies are much more likely to simply bypass AC entirely. It can be annoying as the DM, but no more so than any of a thousand other player strategies. And this one has a very simple solution.
