I was refused once at the gate for F for wearing jeans - I had simply forgot, flying on a BP1.
Once on PanAm, the ticket agent told me to put on a tie before he would give me a boarding pass, flying on a P3.
Used to be you could always identify the airline employees - they were dressed very well including the kids, they got on at the last minute, and carrying all their luggage.
When the UAL pilots had brown uniforms in the 80's, they would stand out at the gate in civvies cause they would have a brown sport coat, brown shoes, etc. The PAA pilots usually had blue blazers, etc.
Even SWA used to have a dress code (and maybe still do) for pilots riding in the cockpit jumpseat not in uniform. I think it was no tennis shoes and no denim.
When I first started flying, the rule of thumb flying international was to dress nice with blazer and tie, cause when they were surveying the line to upgrade Y's to F (pre ff programs) the agent would pick out the pax that were nicely dressed.