Lol, sure. That's why they make stores promise to never, ever, under any circumstances make interchange fees visible to customers.
Hiding in the shadows is not the sign of voluntary mutual-benefit value creation, it's the sign of a good hustle that someone is hoping to milk just a little bit more before the Sauron's Eye of public attention turns their way.
But usually only in small individual shops
If I had to guess, it's still forbidden, but small shops don't know/care until the CC company actually asks them to stop.
If people could get a 5% discount everywhere with their debit card, the gig would be up, and the CC companies know it.
Credit card companies would get bowled over in a second if we didn't let them leverage their size into anticompetitive contract terms, but this is America! We love oligopolies, low-competition markets, anticompetitive behavior, and rent-seeking. Freedom, baby!