I’ve explained it several times now, but I know the conversation can be hard to follow in a fast-moving thread like this. If a player playing a druid character declares that they don metal armor, the only ways for the DM to enforce the statement that druids “will not” wear metal armor is to tell the player “no, you don’t don that armor, because your character wouldn’t do that.” “Your character wouldn’t do that” is pretty much the ur-example of a DM stepping on player agency.
You could of course preserve player agency by creating a house-ruled consequence for donning metal armor, or by relying on the social contract, perhaps with a table rule. But in these cases you’re going outside of the rule itself. Hence my assertion that if you interpret it as a rule, rather than a statement about the lore of the game world, it is either an incomplete rule, or a rule that violates player agency (or if you like, it forces the DM to violate player agency to enforce it.)