If they were widespread they would be obvious to anyone.
My experience is that where there is a Travellers encampment these notices spring up like toadstools.

There have been traveller encampments near me in Midlothian for most of the last 13 years in the summer (the council ordered the eviction of one last year and I haven't seen another one this summer). I have never seen such a sign in a Midlothian pub.

I had a call from a friend confirming that they were common in Glasgow BTW

I spent five years in the 1980s in the east end of Glasgow (where pretty much any kind of bigotry you could imagine was on open display) and never saw one. Whichever foot the regulars kicked with.

Travellers face horrific discrimination in the UK. It is UTTERLY FUCKING STUPID to focus your energies on a mythical wrong when there are so many real ones. "No Travellers" signs in pubs are NOT AN ISSUE, get it?