I mean, we know that offense outstrips defense in 5e. I haven't seen super high AC's, to be honest, things tend to get to 20-21 and stay there. I did see a Fighter with a Displacer Cloak, and he was a menace to deal with.
But my Halfling archer had something like +12 to hit. There was an adventure where we had disadvantage on ranged attack rolls for much of it due to weather conditions, and I never missed a shot, much to the DM's annoyance. In fact, I was still rolling with disadvantage long after he told us to! I mean, it cost me my Sneak Attack damage, but that wasn't really what mattered (Battlemaster 7/Rogue 5), it was mostly my use of Menacing and Commander's Strike.
Sure, in theory, I guess you could have an Eldritch Knight with +3 plate, a +3 shield, and using a Staff of Power for a truly ludicrous 29 AC with Defensive Fighting Style, but that's a pretty extreme example of a high tier character.
Most things don't have that kind of AC, so anything beyond +1 to +2 to hit really feels unnecessary. Armors of more than +2 even more so.
I mean, I only had an AC of 17. No magic armor for me! But it still irritated my DM, because every time he decided he was going to have enemies go out of their way to pressure the pesky Halfling, he was chagrined to realize I had 118 hit points and could Disengage as a bonus action!
I mean do whatever you want, but in my experience PCs with high ACs (higher than 21 or so, or even just 18+ with the ability to cast Shield a couple times) unbalance the game far more than ones with high to-hit bonuses.
What is the difference between a +3 weapon wielded by a PC with a Strength of 29 from a belt of giant strength, and a +7 weapon in the hands of a PC with a strength of 20? Primarily: an attunement slot.
The +3 soft cap is there for a reason, but it is not truly necessary. It is ok to have overpowered PCs, especially at very high levels. I do have a +7 weapon in my campaign, and it has been wielded by PCs in 5E (at the end of two differewnt campaigns) - and it was just fine. It gave the PCs a truly epic retirement adventure experience.
Personally I don't use the standard Gauntlets of Ogre Power or Girdles of Giant Strength precisely because they are as problematic as +4 weapons. I sort of smooshed together the 3e rules and Mutants & Mastermind super strength concept. So Gauntlets of Ogre Power grant +2 to strength (max 19) but the wearer is treated as a large creature for carrying (double capacity) and special abilities like shoving. Girdles of Giant Strength grant +4 (max dependent on the type of belt) but the wearer is treated as a huge creature for carrying (triple capacity) and special abilities like shoving. The means you don't get wierd combos if a min-maxer plugs a dumped stat, it reduces the risk of breaking the maths, but it still feels like you have giant strength when you can lift three times as much and shove a dragon.
I also don't allow magical bonuses from armour and shields to stack.
I have an artifact in my game known as The Sword of No Name*. It is a +6 Defender - you can split the bonus between AC and To Hit/Damage as you see fit on your turn. Lots of other abilities as well (including the ability to dominate the wielders of twelve lesser copies of itself, ala the One Ring).
It's a unique item, not likely to ever fall into the PCs hands. But it just goes to show I'm willing to take the bonus above +3 for some really mythic weapons or items.
* It has a secret name, but that's its common moniker.
I don't think it helps to be too literal with the rules (as decades of debate over hit points and inspirational healing demonstrate). Magical bonuses to armour and shields could represent so many different things, harder material, magical forcefields, absorbing damage, dispersing energy etc. So yeah, for me, the lack of any requirement to attune to basic armour and shields could mean that they break the system. Admittedly, +3 armour and shields are not exactly common but you would be talking AC27 with plate armour and defence fighting style. AC24 but with the option for AC16 if caught without your armour on is still pretty sweet to me and not game breaking.