Like anything from Amazon, movies and TV shows can become out of stock, even when they're digital content. However, some of Amazon Prime Video's offerings disappear on a monthly basis on purpose, planned through contracts with film and television distributors.
On March 22, series stars Jared Padalecki, Jensen Ackles, and Misha Collins announced that the 20-episode fifteenth season of Supernatural would also be its last, making it the longest-running series on The CW.