Why was this mission cut from Metal Gear Solid 5: The Phantom Pain?

Metal Gear Solid 5: The Phantom Pain once had a Mission 51. What happened to it? The PS4 Collector's Edition of Metal Gear Solid 5: The Phantom Pain comes with a Blu-ray stacked with bonus content. According to YouTuber Inzaa, these materials include a look at a cut mission which had been intended to serve