I doubt strongly whether "Travellers by appointment only" has anything to do with "travellers" in the sense of gypsies, Roma, The Travelling People, &c [what sort of "appointments" would any of these be required to make, & for what purpose? to get served in the bar -- oh, come on!].

It clearly refers to sales reps, or, in earlier parlance, "commercial travellers", who would call, often, unless pre-empted by an appointments system, at inconvenient times, trying to sell particular branded drinks &c to the publican —— Represented in popular memory by Clarence Wright's "Good morning! Nice day!" character in Tommy Handley's popular ITMA* comic programme of the 1940s.

I don't deny that there have been many offensive notices on pub doors regarding who wasn't welcome within. But this is clearly not an example of one of them.

~M~

*= "It's That Man Again". Nothing to do with the Irish Traditional Music Association ~~ I recall a thread on that somewhat unfortunate confusion not all that long since!