I’ve been flying for about 25 years now. I always wear at least khakis and a polo and bring a light jacket. Partly b/c I think you should get dressed up a bit when your flying, but mostly b/c I’m always cold. If you want to wear PJs or a wifebeater in first, I don’t agree with it, but thats my issue not yours. Wear whatever you want. I’m not going to lose any sleep over it, and until UA tries to enforce some kind of dress code, they’re not losing any sleep over it either. (Although I do think employees traveling non-rev regardless of cabin have to wear khakis at a minimum, don’t quote me on that but I thought I was told that once by a non-rev who was a seat-mate for a flight)