It doesn't follow the same syntax or vocabulary as ANY other rule. It's a rule that breaks the rules on rule writing. It doesn't sound or look like a rule at all. Any game designer worth his salt wouldn't have added a rule that is so... alien to the rest of the ruleset.
And yet, it is there. You do not like the implementation of the rule, which is ... totally cool! I get it. I'm not arguing with you that you do not like the way the rule is written.* But just because you don't like a rule, doesn't make it ... not a rule.
And when asked, the method to
not have the rule is the same as it is for anything else, talk to your DM-
"If you feel strongly about your druid breaking the taboo and donning metal, talk to your DM."
*As an exercise-
Would it be better if the rule said, "Can not"? "Shall not"? No?
Okay, what if the rule was written with consequences? If it had the worst consequences ever, would that make a difference to you? If the effect of wearing metal armor was the same as getting hit with a (no save) disintegrate spell for 500 hit points, is that better? Or do people want manageable consequences?
Or, would you prefer detailed rules about a "taboo breaker" Druid? Like the oath breaker paladin? Or just rules for losing all your powers?
I'm saying this to point out that there are two basic issues:
A. I don't like the rule
qua rule (I think that Druids should be able to wear metal armor).
B. I don't like the rule's implementation (I don't like the way the rule is worded).
A & B are different. I think that there are people who see the restriction on druids, which carries through- it includes multiclass, feats, whatever. They don't like A. They want the ability to wear metal armor. And they conflate that with B (they don't like the implementation).
Now, there are other people that are just saying "I don't think it's a rule," but ... you know, I can't help them. At a minimum, even if you truly believe it's not a rule for ... reasons, I'm not sure how you can not understand why other people view it as a rule.
shrug