What is a "loading capacitor for pin parameter measurement?"
This is the capacitance the pin is loaded with before making switching speed measurements. It is intended to represent the sort of loading you would get with a length of PCB track and a few gate inputs, so that you are able to get the specified switching times when using a practical board and fanout. If the manufacturer measured the pin unloaded, then you could never achieve the data sheet switching times with any practical board.
It's not a recommendation of how much capacitance should be on the pin for it to function. It's not even an upper limit of how much can be on the pin, though more capacitance than 50 pF will mean that the pin may not meet the switching times mentioned in the data sheet.