D&D Playtester for WoTC since 2012
Right and this is mentionned in Sage Advice, not a rulebook. There is no rules against a druid wearing metal armor.(Also, sage advice is not rules to begin with.)
Also, let's not omit this bit:
Sage Advice: If you want to depart from your class’s story, your DM has the final say on how far you can go and still be considered a member of the class.
I.e. it is a rule, and you need GM's permission to ignore it.
Of corse DM can give consequences, no one need a Sage Advice or any other internet ressource to tell us that. Such consequences would be up to the DM, not a rule in the PHB.