Except that all of those have ACTUAL MECHANICS (well except the secret language things but those have actual in-universe rules applied with proper explanations!).
If you want Druids to not wear metal armor, then just... ya know, make an ACTUAL RULE that makes it a bad idea. make them lose a class feature if they were armor with higher AC bonus than Hide. It's not that difficult, the Monk and Barbarian have all of that right there! Same with the Rogue's weapon restriction.
Those are REAL rule, not some 'just because!' Gygaxian BS. Bleh.