Well, first it was bugged:
What happend to the new mail icon on char screen? - Support / Customer Support - World of Warcraft Forums (blizzard.com)
Then it was turned off because of performance issues (I would call that a “bug”):
Mail icon at Character screen - Support / Customer Support - World of Warcraft Forums (blizzard.com)
Then it was turned off on purpose, but it’s not bugged!
What happend to the new mail icon on char screen? - Support / Customer Support - World of Warcraft Forums (blizzard.com)