To me it's the fact that extracting and refining significant enough quantities of metal to fashion armor isn't easy. It reeks of civilization.
I'm totally fine with someone having that interpretation, but I'm sort of just pointing at the fact that each Druid is an individual, so it would be silly for "no metal" to be a universal concept in a world where it has no penalties. "No metal" means no gold or silver armor, even though you can literally grab it right from nature. Considering the fact that it can even be shaped with magic, and Druids literally have spells like "Heat Metal", I don't see why metal armor made from magical means using natural materials would really be any more a symbol of "civilization" than a Druid using Shillelagh on a stick. In my eyes, it would be totally understandable for a Druid in a grove to have a taboo against wearing metal, because they've been taught that all their life, but then as they travel the world, learn how metal is crafted and used, and consider that it's not as "unnatural" as they once thought.
Leather, by comparison, is fairly easy to fashion. While I'm not a hunter myself, I have it on good authority from a friend of mine who is that basically all you need to tan a deer hide is the deer's brain. (But please keep in mind that there can be prions in the brain that can lead to death if someone isn't careful when using this method, so he also told me that it's not something he'd recommend to anyone in the modern day.)
Well that falls within the point that I made before, in that leather is not made conceptually different, since they're both just merging and refining natural materials, one is just easier. Druids are also allowed to wear very high quality "masterwork" (I know it's not a term anymore, but conceptually I'm sure it still applies in lore) armors, which would reek of civilization as well, as they'd require exceptional tanning methods and craftsmanship to make, which would not appear "primitive" at all.
IMO, it's not so much about metal being bad, since druids can use metal tools, as what armor represents which is civilization. Uncivilized tribes are capable of fashioning leather, but armor made of metal would be largely beyond their means.
Sure, but then that becomes sketchy at best in the RAW when a Druid can dual wield a warhammer and a light hammer, or a battleaxe and a handaxe, regardless of materials. Heck, a Druid can take a feat and dual wield warhammers, but they have taboo against a warhammer and a metal shield. Druids can also gain proficiency in smith's tools, and perform their own blacksmithing, so they can make a metal shield, they just can't wield what they've made.
I wanted to clarify, since my previous post was a bit rushed (we were in the middle of a game session but had paused to grab some dinner).
A druid IMC can worship and serve a god, however it's somewhat like a fighter who serves a god. Unlike a nature cleric, whose power flows from their god, a druids power flows from the primal forces that the god embodies.
It's like a rushing mountain stream that feeds into a calm lake. It's the same water essentially, but the druid is drinking from the stream whereas the cleric drinks from the lake. The druid is closer to the source.
And again, I agree that's all fine. You have a world fleshed out and it has its mechanics. That's exactly why the Sage Advice specifies that players should still ask the DM, because they might undermine their story if they disagree with the implications of their response that nothing in the game system prevents it. My only issue with that aspect is that people assume the whole "make sure you verify with the DM that you're not undermining their story" means that the taboo is a mechanical restriction, and not story fluff, when all they're doing is reminding the reader that you can't just throw the rules at the DM and force them to adhere by them.
If the DM had a reason that Wizards do not exist in their world, then nobody can make a Wizard. I'd never say a single word about that to a DM, except maybe question the implications that has with other magic classes that may be similar, but I will contest the claim that something is RAW when I don't believe it is.
I see what you mean about the PHB saying that druids can gain their magic from a nature deity. That's a bit strange to me, in that why is the druid distinct from the nature cleric when their power comes from the same source. I suppose if a druid player wanted to gain their power from a nature deity, I would have them worship the old aspect of the god, whereas the nature clerics would worship the newer, more humanized face of the deity. Sort of like how the Greek primordial deity Gaia became the goddess Terra in the Roman pantheon.
I would agree it's strange if we assume the 1E lore, but the classes have simply changed. Now Clerics don't actually have a mechanical requirement to pick a deity at all, and Paladins are more like knights than divine agents. As per the fluff in the rules, a Druid can now essentially be considered a Cleric of the Old Faith. In this edition you can get the same power from different sources, or different powers from the same source. You could be a true neutral Cleric that worships a good deity that grants the War domain, and focus your magic on things like Spiritual Weapon and Spirit Guardians, or choose that same deity and gain the Life domain, focusing your magic on healing and restoration and never once learning attack magic.
The classes and races are no longer shoehorned into one concept, not even ones that otherwise make all of the same mechanical choices. It's fine if a DM wants to say they are, but as per the RAW of the core material, it's not the case. That's the only point I'm making.
I wouldn't have dryads coming out of the woodwork to attack the party you've described (unless for some reason the dryads were hyper-aggressive and inclined to attack anyone they felt distrustful of). However, with most druids the dryads would be favorably inclined. Being fey, the dryads are closer to the primal source than most, and they can sense a kinship of sorts with the druid. Think of it as a reaction roll that defaults to a favorable result. In the case of a metal armored druid however, it's like seeing someone you recognize as a soldier for your side wearing the enemy's colors. Automatic unfavorable reaction. They won't necessarily attack, unless it's in their nature to do so, but they certainly aren't likely to help the druid either. As for the cleric and the paladin, that's pretty much just a normal reaction roll. Despite their relationship with nature, they aren't as close to the primal as either the dryad or the druid. They basically have a foot in each world, essentially a serving as intermediaries between the primal and civilized worlds. Basically, the dryad will default to assuming that a non-metal wearing druid is on her side, but she's going to be less sure about the other two. Maybe they'll choose her side or maybe they'll favor the townsfolk. Obviously, this all precludes pre-existing reputations and relationships.
My only concern is that DMs tend to automatically make these creatures know they're a Druid, even when they try to make the claim that "Cleric" and "Druid" are just character sheet titles, and not what NPCs will refer to you as, unless you self-proclaim to be such, like a knight, priest, or samurai likely would. For a silly example, a Nature Cleric, Oath of the Ancients Paladin, and a Druid all walk into a bar, wearing their nature ordained half plates/full plates and metal shields that the Druid himself crafted with his smith's tools proficiency. They present themselves as the "Fey Knights", which is apparently a common title for Oath of the Ancients Paladins, and the whole team likes it so they adopt it. A Dryad walks over and spits in the cup of the Druid, because, "Screw that guy. I sense he's way too classy to be wearing all that nonsense." Personally I feel like the Dryad would assume that person to have the deepest connection to nature out of the three, thus making them the most trustworthy, as opposed to having immediate distrust, but I suppose ultimately it's a DM call.
Just out of curiosity though, in your campaign, if the Druid was aware that a Dryad would not be as open to a Druid in metal, what if the Druid knew he was going to have to interact with a Dryad very soon, and removes the metal out of respect prior to the interaction? Would the Dryad know they've went against the taboo of their order? If not I feel like that penalty could typically be circumvented with some proper planning and forethought, and the Druid could even take note of this and let the Rogue or Bard in the party actively disguise the Druid during typical travels so as not to offend any mystical woodland beings. The Druid doesn't have to feel shame, but I feel it would be like a Tiefling wearing a cloak because they believe other humanoids would distrust them.
But at the end of the day, I don't make it too hard to create armor made from alternate materials, and I don't recall ever having had a DM who did so either, which makes it largely a moot point. It usually involves a mini quest to track down a craftsman and/or a creature with a sufficiently tough hide. Plenty of times though, the PCs come across a proper creature simply as part of their adventures. If there is a craftsman in the party, they might not even need to locate an NPC. So the level of effort might vary a bit, but it's generally something that you can accomplish within the first few levels with a little help from the party.
My issue is that something like "hunting down a creature with sufficiently tough hide" sounds like something more opposed to the nature of a typical Druid than tracking down some iron ore. Most descriptions of Druid say they typically only hunt for survival or self preservation, and I don't find wanting stronger armor to quite fit so well into either of those categories. I could understand if they just so happened to have to take such a creature out, and utilized the hide after killing it, but to hunt something purely for its hide sounds very not Druid to me.
This ignores the fact that I find that following the Druidic lore exactly makes it unlikely the Druid would ever be an adventurer for more than one story arc to begin with, since if they're so typically neutral in alignment and against the tenets of civilization and are so for protecting nature, it sounds odd they'd ever participate in some entry level adventure like being paid by some king to stop goblins from attacking some villages. I feel like a Druid that is not an exception to their order would most likely be completely disinterested in such a trivial thing unless the DM made sure to add an extra hook that made it bad for the balance of nature. The Druid has to be an exception to the rule in some way, otherwise they quite frankly just don't give a damn about anything the other player characters care about, unless those characters also just want to protect the natural balance.