They are? I don't know to what degree this is changed in 2024, but in 2014 these are the various armor proficiencies around:
None: Monk, Sorcerer, Wizard
Mentioning these classes seems like an aside. I'll point out that they are going to invest in DEX regardless for the AC, however, instead of STR and that the Mountain Dwarf added medium armor for wizards and sorcerers who wanted to gain that AC in 2014.
The 2024 rules no longer grant the armor training that way.
Monks are irrelevant to any armor training discussion.
Light: Bard (basic), Rogue, Warlock (basic),
Again, all 3 off these classes are going to be using DEX for an AC bonus regardless of armor. Because they are already investing in DEX for AC they are likely using finesse weapons. DEX is also something they are inclined to for Stealth profiicency.
They can, however, spend take a 13 DEX and spend a single feat to round that DEX to 14 for the max DEX bonus to medium armor while relying on CHA based attacks for the bard or warlock as a smaller investment than trying to pump AC in light armor through ASI's.
Heavy armor, OTOH, would require 2 feats and a higher STR investment for a minor potential in AC bonus. Medium armor is a decent choice for those classes.
A non-DEX rogue is possible but the exception instead of the rule.
Medium: Artificer (basic), Barbarian, Bard (Swords, Valor), Cleric (basic), Druid, Ranger, Warlock (Hexblade)
These are the main medium armor users, yes. In order.....
Artificer: Artificers have a choice of investing directly into DEX or using an infusion to cover it instead. Investing in DEX is a smaller investment than switching to heavy armor and investing in STR, although with the Armorer subclass they do have the option to bypass STR req's for armor. I don't believe we can judge the merits of medium armor based on the Armor subclass of artificers, however.
Barbarian: Barbarians are going to invest in DEX for AC, possibly stealth, possibly ranged weapon attacks, and possibly for their unarmored ability to exceed wearing medium armor. Or they can literally spend one feat on Heavily Armored to round up their STR and ignore DEX for that heavy armor. That has it's own other issues, but it's feasible. Barbarians are a class who might not want to invest in DEX because medium armor but DEX is still something they can use. That doesn't change it to a lower investment than the STR investment for heavy armor.
Bard: Valor and Swords bards are likely to be using finesse weapons along with DEX for AC. A STR Valor bard is possible but less likely.
Cleric: STR investment for heavy armor is a tax on clerics. They are investing in STR for heavy armor or DEX for medium armor but they are investing either way. The DEX investment is a smaller investment.
Druid: Druids are no longer trained in medium armor. They are trained in light armor and shields, and have a class option to learn medium armor. DEX and STR can be irrelevant to Wild Shape so there's not much need to invest in either if using Wild Shape for combat, but similar to basic bards the druid can take Magician as the class feature, make that small investment of 13 DEX, and round it up with Moderately Armored. Or take the Warden feature and invest in the 14 DEX comfortably. Or they can take Warden then spend the feat on Heavily Armored if they really want but that seems like a larger investment over a small gain.
Ranger: DEX is the norm for the ranger. STR is the exception. DEX is used for stealth, ranged weapons, and AC. Rangers, like rogues, are typically well invested into DEX.
Warlock: The Hexblade doesn't exist in the 2024 rules but a warlock with the Blade Pact is in the same category as the basic bard or druid. It's a smaller investment in DEX that they can use in other ways than it is for STR or to get heavy armor. They'll invest in DEX for AC regardless of light or medium armor, or Mage Armor.
Heavy armor requires a heavier investment for almost all of these classes and STR is not a priority for most of them. Many of them do use DEX skill proficiencies like stealth or sleight-of-hand or disarming traps or opening locks in addition to weapon attacks and AC.
Heavy: Artificer (Armorer), Cleric (Forge, Life, Nature, Order, Tempest, Twilight, War), Fighter, Paladin.
That's hardly any of the classes TBH. I mentioned the Armorer artificer already and they're honestly more likely to still use DEX for locks and traps than STR.
Those cleric domains no longer grant heavy armor proficiency. That's been delegated to a choice within the base class and was also covered above.
People were talking about the STR based ranger but what about the DEX based Paladin? These are also applicable builds.
A DEX based fighter is also a very competitive build. Fighters can use DEX for skills, AC, attack rolls, and the DC's for the saves against their abilities. DEX helps them very much.
In this general group fighters and paladins who use STR aren't going to care about any DEX investment for medium armor, and if they use DEX they aren't going to care about any DEX investment for medium armor (or light or no armor) but they'll care about needing to invest STR into heavy armor.
We still can't judge heavy or medium armor based on one subclass from an additional source book for a class that's also not in the core source books when it comes to the Armorer.
The only Medium armor-wearer that is naturally Dex-focused is the ranger (even a melee ranger likes Dex for stealth and such). The two martial bards and the hexblade can certainly be Dex-based, but it's not a core part of the subclass, and one could argue that the intent of giving the subclasses Medium armor is so they can have decent-but-not-great AC without much Dexterity.
Rangers definitely lean into DEX but more martial bards also lean into DEX builds for reasons other than just AC. Look at the College of Dance. That bard isn't likely to wear any armor but DEX for AC and attacks are still going to be important.
It's not a "decent-but-not-great AC". The AC difference with that small investment is 1 point.
