It's not just the diesel that lowers payload capacity -- crew cab, 4x4, long-bed all add a lot of weight that takes away from the left-over payload capacity. Even the DRW option can lower payload capacity on some pickups because it adds weight and it's usually not the tires that are the limiting factor.
Going to a long bed crew cab on a 2020 F350 diesel will actually increase the load capacity beyond a short bed crew cab.
Each brand and truck model is different. It really pays to do research before buying.
