The bad news is that it looks like Konami has no plans on completing it or releasing it to gamers. According to Konami, they claim that the footage shown on the disc was just to give gamers some behind-the-scenes content to show how the game was developed, and not necessarily to act as a precursor for future content.
This is quite a bummer as websites such as Kotaku have claimed that Mission 51 is actually a very important part of the story and is supposed to act as a resolution. However according to Konami, “The ending is found in the missions within. Many threads continue in Metal Gear and subsequent games.”
From what we can tell, it sounds like Konami really will not be releasing Mission 51 to players so if you are planning on picking up Metal Gear Solid 5: The Definitive Experience, don’t expect to find Mission 51 in it either.