What a mess this has all been.
At this point, the long saga of Metal Gear Solid 5: The Phantom Pain, and its incomplete state, prompting wild guesswork and theories about exactly what the mythical Episode 51 and Chapter 3 may entail, and whether or not they would be coming in the first place.
Recently, Konami put an end to all that when they confirmed that no new content would be in Metal Gear Solid 5: Definitive Edition, and that no more content is left for the game- but it appears as though that may not have always been the plan. YouTuber YongYea recently revealed in a video that an unnamed source – he was very careful not to divulge their identity – had confirmed to him that Episode 51 was originally going to be post launch DLC for the game. YongYea speculates that the plan may have been for Kojima to stay on at Konami and make more content for The Phantom Pain after its launch.
I’d say that you should take this with a liberal helping of salt. Whether or not this is true is unknown, although YongYea is generally trusted within the Metal Gear community – but one thing is clear, the final chapter in the Metal Gear saga is incomplete, and is likely to remain so forever.