It's a parenthetical aside that has been repeatedly clarified as a taboo, a choice, and a class flavor. The Sage Advice answer about it even said it's absolutely fine for a druid to wear metal armor if the DM allows it!
Meanwhile, the description for Sneak Attack is actually written in the text. If you don't use a Finesse or Light weapon, you can't get Sneak Attack damage. What are the RAW penalties for a druid who wears metal armor?
It's not a parenthetical aside and in the PHB it's not clarified at all [edit: as a taboo]. It's right there in the proficiency line. It makes sense that it's in parenthesis because it's an exception to the proficiency rule and it breaks the normal structure of the proficiency line.
Sage advice does call it a taboo but says nothing about it being optional. All rules can be modified if the DM allows it, it's called house rules. House rules are perfectly fine. For example, I would let a rogue sneak attack with a greatsword. However, I don't claim that sneak attacking with any weapon you are proficient with is a rule, it's a house rule.
I don't care how you run your game. Pretending it's not "really" a rule because you don't like it is what I don't get.