Table rules and rules as written are not comparable discussions, so that doesn't really require addressing.Agsin, the rule is that druids will not wear metal armor.
If there was a rule at the table which said fire wizards eont become ice wizards, that would apply too.
If the table rule is " PC will not attack other PCs " would that be fluff your character can choose to change their mind on mid-scene when convenient?
As for the rule that Druids will not wear metal armor, it's a rule about character choice that has no penalty for ignoring. A rule denoting character choice, and a rule that places physical restrictions, are two different things in Dungeons and Dragons.
Paladins swear an oath, but they can choose to break their tenets any time they want, because nothing in the game system stops them. Warlocks form a pact with a greater being, but they can tell that being to piss off, because nothing in the system says they have to keep their end of the pact. Clerics worship a deity, but nothing in the mechanics says they have to, so they can tell the gods to piss off. Druids have a taboo against metal armor, but nothing in the system prevents them from having the ability to don metal armor. With the exception of the Paladin, none of these have any mechanical implications listed in the books, and thus any penalties applied would be house rules.
We're going on 5 years into the release of 5E, and have had 10 separate printings of the PHB. Sage Advice addressed this issue over 3 years ago. If Druids were intended to have a mechanic that prevented them from having the ability to wear metal armor, as opposed to just having a taboo against it, it would have been written into the game system by now. Instead the lead rules developer has given an official response confirming that nothing in the game system stops them if they choose to do so, and that the only limitation would be if the DM felt it didn't fit in their story, which is just a reminder of something that is true for everything in the game.