Is salt OP?

definitely. But i dont see it too much really

Another advantage with salt is that unlike other luxuries it is not spread out across the map and it is not available through cs meaning that it will be way easier to control the salt trade.

Salt is not alone. Play as the Huns and get start with several cattle / sheep / horses. Starting with animal husbandry and auto +1 production from pastures makes cattle immediately give 3 food 2 production once improved (same as plains salt without gold). Build a stable, 3 food 3 production very early, and eventually get 4 & 3 with fertiliser. I don't know why more people don't mention the Huns as a great versatile civ, as this early bonus is one I have found to be very powerful.