The slogan "We reserve the right to refuse service to anyone" is the classic sign used to tell Blacks they were not welcome.
I have not seen such a sign in decades. I think you may only have seen it in your imagination.
Putting up such a sign is not illegal. Refusing to serve someone based on race, gender or whatever is clearly illegal. Most people do not sue over such bigotry, they just go to any local chain restaurant (Denny's, Chili's, McDonald's, BK, wherever) that does not post such signs.
I have seen "No shoes, no shirt, no service", but that all I have seen lately.
I can't recall seeing barefoot or shirtless people anywhere except on the beach in any restaurant.