the >'05 cars had a diff that freewheels until power is applied to the electromagnet that locks the clutchpack inside the diff, thus connecting the front and rear for torque transfer. Inserting the fuse in the front on those cars applies power to a relay, which then OPENS the circuit to the DCCD in the transmission causing it to fully unlock and apply 100% torque to the front wheels.
'06+ the DCCD has a spider transfer section and cannot be completely unlocked even if power is cut to the electromagnet, thus no fuse and no FWD in those cars.
NOTE this is for the STi six speed though. I’m not 100% positive if the five speed from '06 on went with the spider DCCD or not. information that i’ve never needed myself. Thus the WRX/Legacy/etc/etc may indeed have that FWD fuse still