The logical conclusion of this is that the moment somebody gains Druidic spellcasting, they are immediately brainwashed into refusing to wear metal armour under any circumstance. And that this applies to all Druids, like some universal hivemind thing, and is impossible for any of them to ever defy.
If really leaned into, this could be an unsettling setting element. Otherwise it's just silly, and not an accurate depiction of how religious taboos work. Do you think that because Catholics are only allowed to eat one full meal + two snacks on Friday, and that those meals can't include meat, that all Catholics will not eat meat on Friday?