tennesetom's suggestion of a standard for pass riders is useful.
Delta has a relaxed dress code for pass riders, including Buddies. The standard is based on respect — for our customers and for you. Delta trusts your good judgment when traveling on a Buddy Pass. Just remember, Delta has a relaxed dress code for pass riders, but that doesn’t mean a sloppy appearance is acceptable. You should never wear unclean, revealing or lewd garments, or swimwear or sleepwear on a flight. The relaxed dress code also applies for Buddy Pass travel on Delta Connection carriers.
https://thegate.boardingarea.com/bud...lta-air-lines/
As for non-Pass riders, welcome to the U.S.A. of the 21st century. You need footwear. A shirt, and pants or shorts - nothing lewd. No profane proclamations on the garments. That's about it. It used to be detailed in the Contract of Carriage, but they've broken up the CofC and dumbed it down a dozen ways.