The threads talking about studded leather grips for polearms mentioned a couple things that got me thinking. Of the (many) things D&D abstracts, it's the durability of gear and any actions or costs needed to maintain it. Oh sure, the rules talk about what it takes to outright destroy something, but you can wear a suit of armor from level 1 to 20 and probably never have to replace it (unless an upgrade becomes available). I remember an optional system in the 2e Complete Fighter's Handbook for armor durability, but I have to assume it was unpopular- I remember trying to use it in the 90's but giving up on the idea quickly as it was one of those things that was very punishing at the low levels, but higher level characters could afford more durable armors or have access to magic to repair things in the field.
The other thing is helmets. After all, outside of optional rules, D&D doesn't use a hit location system or called shots, so there's not much reason to wear a helmet. Again, the 2e Complete Fighter's Handbook presented a variety of helmets, with their penalties (vision) and benefits (mostly to saves vs. various types of attacks). Because the PHB had costs for helmets, but didn't say what they were for. Oh and I recall the Thief's Detect Noise required you to not wear a helmet, which, as a rule, I don't think I ever saw a Thief wearing one.
And outside of called shots and "critical hit charts", there wasn't much reason to wear a helmet (unless you found a magic one). 3.x at least mentioned that some armor sets came with a helmet and/or gauntlets, but rules for not wearing one? Absent.
Official art hasn't helped much. It often seems you see as many people wearing headgear as not. Ditto for fantasy movies. Which I get from an artistic standpoint- you can't see as much of a character's pretty (or ugly) mug if it's obscured by a helmet, but I'm pretty sure that, as a result, if you ask if a PC is wearing a helmet or not (and they weren't immediately suspicious of your motives, lol, like what happens when I ask if someone is wearing gloves as a DM), they'd probably blink at you and say "uh, why?".
I can accept a lot of abstraction in my fantasy games, especially if the juice isn't worth the squeeze (like my recent experiment of making my players pony up cash for very abstract "supplies" when traveling- it took them til about 7th level, but they finally realized they could break the system and never need worry about it again, outside of something outright destroying their supplies, which happened once when they fought a black dragon, as it had a lair action to spoil foodstuffs, foul water, and corrode gear). So maybe there isn't a reason to track wear and tear to equipment, or worry about people wearing helmets, but it's also something I'm wondering about.
What would gear degradation look like? And is 5e granular enough to accept rules for helmets, so that there's a clear difference between wearing a chain coif or a great helm that isn't strictly punitive?
IE, something like "oh you're not wearing a helmet, ok, you can be crit on a 19-20. Oh you are wearing a helmet? Disadvantage on Perception checks!"- that seems harsh.