It literally says straight out in the PHB that that's exactly how they get their powers, kid. As I mentioned in the post you quoted.
PHB, p64 under heading Power of Nature, 1st paragraph:
"Druids revere nature above all, gaining their spells and other magical powers either through the force of nature itself [a deific power] or from a nature deity."
Your mistake here is assuming that "the force of nature itself" means "deity" it doesn't.
The same consequence a Paladin suffers for breaking their oath, a Warlock does for failing to live up to the terms of their pact, a Cleric for making a serious blasphemy or even turning away entirely from their deity or deific force - whatever the DM decides it is. The default presumption is not "there are no consequences". Pretty sure this has come up at least 80 times in this thread.
But "there are no consequences" is a legitimate answer. If a Devotion Paladin lies to protect children hiding in the closet from being killed, he may feel shame that he didn't live up to the highest ideals of his order, but his shame being weighed against the lives of two children... well, let's just say that I would have a major problem with any force that said the paladin should lose their abilities for not "doing the right thing"
A Warlock "failing to live up to their pact" is very loose language. And Warlocks defying their patrons and their pacts is a pretty old tale. And sometimes, a lot of times, they don't lose their powers.
Clerics turning against their gods? I actually wish I could remember the name of the novel, but it was an official Forgotten Realms novel. Featured a theif/cleric. Guy was chosen to be a Cleric by Mask. Did he worship Mask? No. Did he like Mask? No. Did he defy Mask at every turn? Yes. Did it matter one tiny little bit? Nope!
The idea of a cleric being forced to serve a god because the God says so is another concept that has a lot of traction. Enough that I can't even find the novel I'm thinking of.
So, sure, "The DM decides" but can you give me a reason that the DM
should decide anything happens? I mean, most Fiend Warlocks are expecting to betray their patrons and still retain their powers, which you say is the same sort of taboo that wearing metal would be for a druid... so why should the druid expect anything to happen to them if they did? If I can play a Cleric who curses his Diety's name.... why can't I play a Druid who wears metal and uses a metal shield?
The magic of the oldest druidic sect in Eberron, the Gatekeepers (as taught by VVaraak), stemmed from bonds between the three progenitor dragons. Which, sorry to burst your bubble, are divine beings. It's implied that some other sects might derive their abilities from things like fey (e.g Greensingers), which would be something like animism, but never directly stated to my knowledge. There's most definitely no implication that the Metal Armor prohibition doesn't exist there.
That's weird. Looking over my notes on Eberron, specifically the stuff I took from Keith Baker's blog, there is no indication that Syberis, Eberron and Khyber were real and not just a metaphor for the power above the planet, the planet and the dangers lurking below the surface of the planet.
So, sorry to burst your bubble, but those are places, not divine beings. And, even if they were, since Eberron is the planet itself, would you like to try and explain why metal (the material of Ebberron) is rejected by... Eberron? Is a self-hatred thing? Seems really weird that if this divine being exists (which again, we literally have no proof of, on purpose by the designer of the place) that they are okay with fire, water, wood, animal skins, air, stone, dirt, bone... but not metal?
I have no access to Ravnica-related campaign materials and thus no way to verify whether your claim is true. The official MTG website (MTG being the progenitor of Ravnica) has some things to say on the matter, however. Per the "Druids, Trees, and Truth" publication: "The druid's power—like all true power—comes from the land. He recognizes that, to gain access the fundamental forces of the world, the first step is the subordination of the self to the will of nature." That describes worship / spiritual veneration. Making "Nature" a deific force if not technically a god. You're free to write or rewrite WHATEVER you choose in your homebrew setting, of course.
Cool, cool, so "the will of nature" could be completely mindless and just acting to grow and thrive and survive. I mean, I'm sure the "will of nature" in a forest would be that a mudslide that destroys the forest is bad, but that doesn't stop the rain from weakening a mountainside and causing that mudslide anyways.
"The Will of Nature" isn't a god, it isn't a single unified force. That just doesn't make sense in the context of the situation.
So, again, your assertion that all druids draw their power from some "deity" in some manner is just wrong.
So much nothing that the prohibition against metal armor he instituted in 1e persisted almost unchanged through all the succeeding editions he wasn't involved with until 4e at least (about which I don't know because I couldn't stand 4e); with the slight evolution from "can't wear metal armor" to "loses all magical abilities for 24 hours if they put on metal armor". And even in 5e the prohibition remains - but again, as with all conduct violations in 5e, the consequences are left up to the DM.
Just because people kept the name "rogue" doesn't mean that they are identical in 5e to 2e. Gygax's opinion on what he would or would not allow have nothing to do with 5e. In 1e, there was a good balance reason for the restriction, because 1e was designed differently.
There is no balance to the current restriction, and it makes no sense. Metal is a part of nature. If making it hot and bending it makes it unacceptable, then lava would be unacceptable. Or Ice sculptures for that matter. Shaping a tree would be unacceptable.
The specific nature of religious practice doesn't actually matter. A taboo or prohibition exists within a religious context because it is presumed according to that tradition to offend some spirit or deific figure; or to demonstrate some symbolic loyalty; or because violating the taboo is presumed to have some deleterious effect on one's own spiritual nature (be it a soul or karma). In a world where there are direct, tangible, magical benefits received from specific religious practice (i.e. druidic empowerment) - then one is going to have direct, tangible, magical consequences for violating the tenets of that practice. Even if just the (temporary) loss of the aforementioned benefits.
So why does nature hate nature enough to remove the benefits and powers they have granted by using a natural material?