Robinson-Patman is a bad law. Even if it were a good law, it would be effectively impossible to enforce equitably and not capriciously, which is probably why it hasn’t been.

The government telling competitive buyers and sellers which kinds of price negotiation are legal and which are not is terrible economics because it attenuates price signals.

Pre-1980's, it seems to have been enforced well enough to keep a whole lot of mom-and-pop grocery stores in business.

I'm thinking the faded-out enforcement was due to a certain 1980's President, and his administration's "Greed is God" ethos. A lot of protections for the little guy got dumped on his watch.