What do I call this "heel fixture" used for attaching a pallet rack column to the floor?
They are called baseplates.
BASEPLATES
An upright frame has at least (2) baseplates used to anchor the pallet rack to the floor for stability. The size of the baseplate is based on seismic forces and the loads placed on the pallet rack. Engineers should check the design to make sure the proper baseplate is specified for the pallet racking.
UNARCO: Components of a Rack System
I thought it would be called a flange, but it seems that 90-degree mounting plate or mounting bracket is a better fit.