Come on, folks. This "Prove it! Prove it or ELSE!" attitude isn't helpful.
It's in the Player's Handbook, right there next to the druid's armor proficiencies. It's different, but it's there. We can go (and have gone) nuts with discussions of why it's there, and how it got there, and whether or not it should be there at all, but...it's still there.
If you want to play a druid and you have an issue with this (or any other) rule, all you have to do is ask your DM about it and talk it through. Your DM might change the rule and give you an alternative, or they might suggest you pick a different class to play if your character concept is focused on metal armor, it's all good. That's totally cool.
Or you could play the rule as intended: your DM might ask you "So tell us a little about your druid: why won't your druid wear metal armor?" and that would give you a chance to say something cool and personal about your character, something like "my druid is claustrophobic and cannot stand the thought of being encased in inorganic material" or whatever, and the story is richer for it. That's cool too.
But it's not cool for you to say "I'm going to play a druid, but I don't like this one rule so I'm going to ignore it. My druid is going to make a beeline to the armor shop, buy a steel breastplate, and wear it, rules be damned. And then I'm going to argue with you, and the rulebook, and everyone else at this table until everyone agrees with me." Seriously, who does that?