Mycoid Shepherd’s ability will trigger when it’s put into a graveyard from the battlefield no matter what its power is at that time.

If another creature you control is put into a graveyard from the battlefield, Mycoid Shepherd’s ability checks that creature’s power as it last existed on the battlefield. If it was 5 or greater, the ability will trigger.

If Mycoid Shepherd and another creature you control with power 5 or greater are put into a graveyard from the battlefield at the same time, Mycoid Shepherd’s ability will trigger twice.