After the Second Wizarding War, the Ministry of Magic became headed by former-Auror and Order of the Phoenix member Kingsley Shacklebolt. Due to their actions during the war (allowing Death Eaters to escape back to Voldemort and joining forces with him themselves), Shacklebolt disbanded the Dementors as guards of the wizard prison. It is unknown where they went or what they did without their only known application. It is likely that more humane security measures were put in place at Azkaban.