What even is armor proficiency?

[Greywander]

Or would really only wear armour for combat / anticipated combat... which (as you mention) makes sense! If hiking in full plate is cumbersome or more difficult, maybe they wear something else for that task even if a planned major combat deserves (and receives) full plate.

Yeah, like I said, a brigandine + gambeson would probably be one of the more common armor loadouts for adventuring, since it provides pretty decent protection without being too uncomfortable. If you're anticipating combat, then you might upgrade to full plate, but you'll probably strip down to something lighter if you're not expecting to fight. Though if you have full plate, you'll probably just wear the breastplate instead of a brigandine when dressed down, that way you don't have to carry around an extra brigandine.

I'm not fully understanding the problem you're having. Can you provide some specifics as to exactly who is multiclassing into what to gain armor proficiencies?

The problem isn't so much that you can dip, I just brought them up to show how proficiency isn't really effective at limiting who can use armor. A few common such dips are wizards dipping into cleric, sorcerers and bards dipping into Hexblade, any CHA caster dipping into paladin, and anyone dipping into fighter.

My problem has always been that proficiency feels too much like a game abstraction. Like, imagine you have a perfectly normal gun, and you have extra ammo, but the (hypothetical) rules of the game only allow you to reload on a short rest. So how do you roleplay running out of ammo in the gun? The game might be balanced around it, but it would still feel super weird. Armor proficiency feels the same to me. The rules themselves are abstractions, but they still represent a fictional reality. The abstractions work on a high level to determine outcomes and such, but that has to translate back into fictional reality. When I attack, deal damage, and reduce a target to 0 HP, those are all abstractions, but they pretty easily translate back into the fantasy simulation; I hit it with my sword and it died. The abstractions themselves aren't the issue, the problem is when those abstractions dictate an outcome that doesn't translate in a way that makes sense.

People say D&D isn't a fantasy world simulator, and that's both right and wrong. It's not a simulation type of game; it's not interested in using complex rules to determine in detail what happens in the fantasy world. It just tells you whether something succeeds or not, and then you come up with a justification for why it did or didn't work. However, D&D isn't a videogame-esque world. NPCs don't talk about stats or hit points. Characters aren't actually having idle chats while standing perfectly still waiting for the wizard to decide what spell to cast and finish their turn. The settings in which D&D take place are meant to be more realistic settings, where HP, turn-based combat, and so on don't exist. You'll never hear Drizzle Durden talking about which feats he took, or Elfminister talking about class levels. When someone suggests you put on some armor before a big battle, "Oh I'm not trained to fight in armor," is not a response that makes logical sense, not for a career adventurer.

So if armor proficiency doesn't really make sense, and it isn't even that good at stopping people from wearing armor, is there a better way to handle armor restrictions? That's what I'm asking.

Maybe what would work best would be some kind of penalty for wearing armor over a long period of time. For example, every hour you spend in armor you make a CON save against exhaustion with a DC equal to the armor's base AC (e.g. 12 for studded leather, 18 for plate), and you can recover all levels of this exhaustion after a short rest. Those with high CON, and especially CON save proficiency, won't have as much of an issue with these. Those with low CON will be less likely to travel in armor, though they can still put it on if they expect a fight. This at least allows you to armor up when it would make logical sense to do so, but gives you a very good reason not to just wear armor all the time. Might want to move concentration to using WIS instead of CON, though, since otherwise casters will tend to have decent CON.