I recall seeing those signs posted in diners in the 50's and 60's. "We reserve the right to refuse service to anyone." It was considered code for "order take-out and don't ask to use the restroom" if you are a member of a minority group, as I recall.
Have not seen one of those signs in years. But today I was in Ann Arbor, Michigan and spotted one of the signs in WF. I rather expect anyone under 55 or 60 would not have given it a thought, but that sign brought back memories of a less inclusive time.