Personally, a shirt, trousers and shoes - Regardless.
Have been refused by Aer Lingus for a 55 min flight DUB/LHR in Y class.
I take no risks.
United have a dress code. If you wear jeans, you'll be lucky to get Y.

DL must have relaxed their regs, the last time I flew them I was dressed up, ATL/LIM.
Prior to that trip, I hadn't flown them since the late 80's. I had a leather jacket in my hands and I was told to fold it inside out and drape it over
my arms at SFO., that too for Y class.