This is probably a relic of the past–since the only place I’ve seen it is in old movies. When a character needs to get into a restaurant with a dress-code, the maitre d’ will often stop him because he’s underdressed . The comic staple is that the jacket/tie the restaurant offers him to wear before entering is hideous. (the “Friar’s Club” episode of “Seinfeld” is the only example I can think of now, where the club loans Jerry a jacket before he can enter).
Did any place actually have clothing on hand for underdressed diners to wear, or do any dopers know of clubs/restaurants that do this now? My best guess is that the staff would instead just quietly show you the door, and I’ve been in enough jacket-required restaurants that I think I’d have spotted a mis-matched diner by now…