That setup was permitted in the past.
Meaning there was no jumper?
New installations require a separate ground in the feeder.
Meaning a cable with four conductors from main to outbuilding?
It's not my place to come up with a rationale for the electrical code, but:
Option 1, with outbuilding N & G unconnected, a person who is grounded at the outbuilding (let's say standing barefoot on wet ground) would see zero volts on outbuilding metal enclosures.
Option 2, with outbuilding N & G connected, a person who is grounded at the outbuilding would see 2-3 v on outbuilding metal enclosures assuming current was being drawn by the outbuilding.
Option 3, with outbuilding N & G connected through a four wire cable from the main building and with outbuilding N & G unconnected, a person who is grounded at the outbuilding would see the voltage difference between the ground at the outbuilding and the ground at the main building.
This voltage is probably indeterminate and small unless the current density in the ground happens to be very high, like in a metropolitan area.
I'd say go with option 1 but the electrical code people must have some good reason for picking option 3.
These various option voltages can be tested with a multimeter and a heavy 120v load turned on in the outbuilding.