I usually see it, but inverted: "we offer a 4% discount for cash payments" signs

But usually only in small individual shops

If that's allowed under the credit card processing merchant agreements it's a new development.

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.

Not sure why raised public attention would reduce credit card usage. People like the convenience, and I don’t see why they would care that it shaves a few basis points off of merchants’ margins. You’ll notice that in transactions with razor thin margins, credit cards aren’t accepted, restaurants being the major exception.

If people could get "5% off" (really: not paying CC fees) by using a debit card and also use it for rent and government payments, CCs would be gone in a few years. If merchant margins are razor thin, reductions in cost go to the customer, not to the merchant's margins.

I’ll draw an analogy. Most tech firms will make you sign an NDA as a condition of employment. That doesn’t mean that you are forced to work there.

Uh-huh. Much freedom. Very liberty.

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!