I think Delta is (or was, I haven't been on them recently) the only airline that still had a strict rule about pilot attire, including the wearing of a hat (or at least for Captains) at all times the pilot is visible to customers. IIRC, hat and blazer (buttoned) must be worn at all times while in the terminal, or visible to customers during boarding/deplaning.

I haven't seen a hat on a CO pilot in many years...I can't remember when...