OK, let's look at ogres then - which are about when I'd expect to see plate appearing..
- AD&D: THAC0 17 (so 16 to hit field plate + shield), and either 1d10 or [weapon] + 6 damage AC5, 4+1hd (19ish)
- 3.X: +8 to hit (so 12s to hit full plate + shield), 2d8+7 damage. Or +1 to hit at range, d8+5 damage. AC16 29hp
- 5e:+6 to hit (so 14 to hit full plate + shield), 2d8+4 damage melee, 2d6+4 ranged. AC11, 59 (!) hp.
That 5e ogre is a bit of a bullet sponge compared to the rest.
A 5e cloud giant is CR9 and has a +12 to hit with two attacks at 3d8+8. Admittedly a 3.5 cloud giant is rolling at +22/+17/+12 to hit, doing 4d6+18 damage (and a cloud giant has a THAC0 of 5).
Right, but you might as well go all out and do full plate and shield for AC 0, since that is equal to plate and shield (AC 20) in 5E.
Then in AD&D (2E) you need 17 to hit AC 0, assuming d8 weapon would make 2.325 average DPR.
5E you need 14 or better to hit AC 20 for an average DPR of 5 points, over twice the DPR.
Compare that to say Studded Leather and DEX 17.
In AD&D, that would be AC 7 - 3 for AC 4. With the THAC0 17, the ogre needs 13 or better to hit, averaging 4.425 DPR.
In 5E, that is AC 12 + 3 for AC 15. With +6 attack, you only need a 9 to hit, averaging 8.25 DPR. Not quite twice the DPR, but close.
So, yes, armor counts for less (all armor really, but less so with lighter armors), and they counter it by boosting HP (easier CON bonuses in 3rd edition on), more healing to recover from the extra damage after each fight, etc.
And in general they did the same to ACs for the monsters. The ogre went from AC 15 (equivalent to AC 5 in AD&D), to AC 16, to AC 11, but hit points exploded increasing by 50% and then another 100% (19, 29, 59!). Damage output increases in each edition as well overall IME.
The net result, people hit more (yeah??) and get hit more (yeah???) but can take more (more yeah????) and dish it out more (even more yeah?????). Another side effect is since you are getting hit more in later editions, HP have to present abstract concepts even more so than in AD&D.