I need to further clarify (and revise) my position, because I realized something.
Player agency and
character agency are not the same thing.
There are a lot of 'choices' a player could make that I simply wouldn't allow at my table. It's not taking away player agency, it's enforcing the social contract that everyone agreed to.
For example, if a player said "I go find the town merchant, kill them, and take their stuff." I would just say, no, you don't. Before we started the game everyone agreed to not play an evil character and part of the definition of that was doing this sort of stuff.
The same goes for attacking other party members. It's simply not allowed.
How I see it is that all D&D classes come with certain amount of lore that assumes certain actions on part of the characters. Clerics pray to the gods to gain spells, wizards have to study spellbooks etc. And the player knew this when they chose that class. The willingly signed up for it. So that's why I don't think it is a terrible agency violation to expect them to follow these things they willingly chose. The armour restriction is the same. Druids not wearing metal is a part of being the druid, and the player chose to accept this when they made a druid.
I completely agree that
players can be held to choices they made at the start of the game. That's part of the social contract. Choosing to do something you've agreed you aren't going to do is choosing not to maintain that social contract and might be choosing not to play the game. If it involves choosing an actions for your character, the DM is within their responsibility of enforcing the social contract by saying that, no, your character does not do that. You have the choice to change your mind about their actions or leave the game.
A PC druid not wearing metal armor can be part of this social contract. If the
player chooses to play the druid in such a game, they are choosing not to wear metal armor. If an oddball situation comes up (such as my example of temporarily hiding in plate armor for some really good reason), they can discuss it with the DM and see if the DM feels an exception is appropriate or not.
The issue that I'm actually focused on is the ramifications of
character agency. To avoid confusion, consider my reference to
character in context of character agency to equal
NPC.
Let me remind everyone that the Sage Advice likens a Druid wearing metal armour to a vegetarian eating meat. It's a contradiction. It is impossible because it is a conflicting definition.
When it comes to taboos and the (silly IMHO) scenario where a druid has to wear metal armor to save the world ... I have a story.
...
If I was playing a druid and I took my taboo against wearing metal armor seriously, I would feel the same way. While I'm going to plead the 5th on personal religious beliefs or lack therein, people's faith is not something to f*** around with just because you can. The DM would have to knowingly set up this very specific scenario in an attempt to force someone to ignore a taboo, a very fundamental part of their belief system. I don't think it's okay to make lightly of religious beliefs like that.
This is why the distinction between
player and (NPC)
character agency is important.
When I have supported extreme examples it has been to attempt to figure out how someone's
world works; not how their
game works. So, as I said, it's fine if the
player isn't allowed to choose to wear metal armor under any foreseeable condition (or needs to discuss it with the DM if a rare situation comes up) as part of the social contract of the
game. What I need to understand is what is going on
in the world, with
characters (again, stick to NPCs if it helps bring the distinction front and center as I intend it to be).
Have any NPC druid
characters in the theoretical history of the world ever put on metal armor? If not, that seems...really, really, mind-bafflingly odd. I want to understand if anyone is
actually saying that in their game world no hypothetical NPC druid has ever, under any condition, encased themselves in metal, or fallen from their druidic faith (with whatever consequences that entailed). Is anyone really saying that an NPC
character when they become a druid become incapable of ever after violating this taboo?
Some people might be discussing something else, but from what I can tell, the crux of the argument comes down to a failure (which I was also guilty of) to correctly separate the issues of
player agency in the game from
character agency in the world.
(As an aside, I think the horrible phrasing in the PHB is responsible for this confusion, as druid armor is the only place where this confusion comes up.)