Having a reasonable armour class is pretty fundamental to D&D melee combat.
I'm not going to try and unpack the OD&D alternative combat tables. But in Moldvay Basic, here are some chances to be hit by various opponents, for an unarmoured character and one wearing the heaviest armour available (plate mail):
Even against an ogre, plate mail comes close to halving the chance of being hurt.
In AD&D, the numbers are:
Plate mail gives even better protection than in Moldvay Basic.
In 5e, the numbers are:
This is protection similar to Moldvay Basic.
The simple upshot is that unarmoured characters are not really viable in melee combat. But fantasy tropes include unarmoured characters who fight in melee! So some further mechanic needs to be used to boost their AC.
There are different ways of doing this. One is to enhance the DEX bonus for these character types. This is, in effect, what AD&D does for the UA Barbarian (basically, a doubling of the DEX bonus) and the OA Kensai (basically, +1 to the DEX bonus). The 4e Monk is a DEX-based class which also gets a flat AC buff.
Another approach is to grant an attribute-independent, level dependent bonus. This is, in effect, what AD&D does for the Monk (in both the PHB and OA versions) and for the Kensai (special AC bonuses as levels are gained). 4e does something similar for the Barbarian.
In 5e, though, this is handled via a non-DEX attribute bonus: a Barbarian's Unarmored Defense adds CON to AC; a Monk's adds WIS. This creates significant pressure for PCs of those classes to lean into those stats, in order to have viable ACs for melee combat. The result is a reinforcement of stereotypes: "uncivilised" warriors are distinctively tough compared to ordinary warriors; East Asian-style martial artists draw on a wisdom that distinguishes them from their "ordinary" counterparts.
I think the AD&D and 4e approaches to this AC issues are superior. They allow these unarmoured characters to be viable, but leave it to the player to what extent they wish to lean into stereotypes or push against them. For instance, the 4e PHB2 has both a CON-based and a CHA-based Barbarian; the 4e PHB3 has both a WIS-based Monk and a STR-based Monk. A 4e Fighter is as likely to have high WIS (to buff OAs) as a 4e Monk is. There is not the essentialism that seems implicit in the 5e approach.
I'm not going to try and unpack the OD&D alternative combat tables. But in Moldvay Basic, here are some chances to be hit by various opponents, for an unarmoured character and one wearing the heaviest armour available (plate mail):
Unarmoured: normal person 0.5; orc or typical soldier 0.55; ogre 0.75
Wearing plate mail: normal person 0.2; orc or typical soldier 0.25; ogre 0.45
Wearing plate mail: normal person 0.2; orc or typical soldier 0.25; ogre 0.45
Even against an ogre, plate mail comes close to halving the chance of being hurt.
In AD&D, the numbers are:
Unarmoured: normal person or typical soldier 0.5; orc 0.6; ogre 0.8
Wearing plate mail: normal person or typical soldier 0.15; orc 0.25; ogre 0.45
Wearing plate mail: normal person or typical soldier 0.15; orc 0.25; ogre 0.45
Plate mail gives even better protection than in Moldvay Basic.
In 5e, the numbers are:
Unarmoured: commoner 0.65; bandit or guard 0.7; orc 0.8; ogre 0.85
Wearing plate armour: commoner 0.25; bandit or guard 0.3; orc 0.4; ogre 0.45
Wearing plate armour: commoner 0.25; bandit or guard 0.3; orc 0.4; ogre 0.45
This is protection similar to Moldvay Basic.
The simple upshot is that unarmoured characters are not really viable in melee combat. But fantasy tropes include unarmoured characters who fight in melee! So some further mechanic needs to be used to boost their AC.
There are different ways of doing this. One is to enhance the DEX bonus for these character types. This is, in effect, what AD&D does for the UA Barbarian (basically, a doubling of the DEX bonus) and the OA Kensai (basically, +1 to the DEX bonus). The 4e Monk is a DEX-based class which also gets a flat AC buff.
Another approach is to grant an attribute-independent, level dependent bonus. This is, in effect, what AD&D does for the Monk (in both the PHB and OA versions) and for the Kensai (special AC bonuses as levels are gained). 4e does something similar for the Barbarian.
In 5e, though, this is handled via a non-DEX attribute bonus: a Barbarian's Unarmored Defense adds CON to AC; a Monk's adds WIS. This creates significant pressure for PCs of those classes to lean into those stats, in order to have viable ACs for melee combat. The result is a reinforcement of stereotypes: "uncivilised" warriors are distinctively tough compared to ordinary warriors; East Asian-style martial artists draw on a wisdom that distinguishes them from their "ordinary" counterparts.
I think the AD&D and 4e approaches to this AC issues are superior. They allow these unarmoured characters to be viable, but leave it to the player to what extent they wish to lean into stereotypes or push against them. For instance, the 4e PHB2 has both a CON-based and a CHA-based Barbarian; the 4e PHB3 has both a WIS-based Monk and a STR-based Monk. A 4e Fighter is as likely to have high WIS (to buff OAs) as a 4e Monk is. There is not the essentialism that seems implicit in the 5e approach.