Polyjuice potion was outlawed soon after the war ended. Although it had always been an extremely difficult potion to brew and very rarely used, there was a concern that the Resistance or some other faction against the rise of the new regime would dip into the arcane potions volumes to rediscover how to brew it. The Dark Lord had been impressed by the potions use by both his faithful servant Barty Crouch Jr. and Draco's use of it during the year he was ordered to kill Dumbledore. In the wrong hands, there was no end to the amount of damage it could cause.