Konami talks about MGSV’s cut Mission 51: it’s not the ending
The past year, one thing that was constantly a point of discussion when it comes to Metal Gear Solid V: The Phantom Pain is the illusive ‘Mission 51 (Kingdom of the Flies)’, an additional mission that was cut during development. The missions unfinished cutscenes and boss battle were put on the Collector’s Edition bonus disc. and has been a controversial subject ever since. Now, with the official announcement of the Definitive Experience, Konami has responded to some claims and questions regarding this mission.
So, according to Konami, this mission was never supposed to be the ending, but rather an additional mission that happened to get cut somewhere during development. Whether we will ever get to play it remains to be seen, though there seem to be no plans for that right now.
Source: Metal Gear Official Twitter