Description and Acquisition
In the original release of the game once the player has completed the patrol cases, this outfit cannot be worn, unless you replay the patrol cases. In the enhanced versions of the game Rockstar has added the ability to wear the outfit outside of the patrol cases.
Rockstar was taking into consideration the fact that some players want to wear this outfit throughout the game, not just patrol and it may have been considered for a future DLC outfit at one point. However, Rockstar has since announced no new content would be added. 
In the Nintendo Switch, Xbox One and PlayStation 4 versions. The Beat Cop Outfit becomes available after Phelps gets promoted from Patrol to Traffic.