This revelation comes from last year’s The Ultimate History Of Video Games, Volume 2 by Steven L. Kent. The book has quotes from former Konami assistant producer Ryan Payton, who talked about long-running rumours of a Metal Gear Solid exclusivity deal at PlayStation (from ResetEra via The Gamer).
“The prevailing belief around the industry was that Sony must have secured an exclusive deal for the game. It wasn’t true,” explained Payton. “Had Kojima chosen to port the game to Xbox 360, his bosses would surely have approved the decision, but he wasn’t ready to make the jump.”
This reflects quotes from Kojima in 2013, who said at the time that the Metal Gear Solid: Legacy Collection wouldn’t come to Xbox 360 due to the disc storage size (via Eurogamer).
“The amount of data in Metal Gear Solid 4 is just too enormous,” said Kojima. “I want to put it out, but we won’t be releasing it. When it’s the next [Xbox] console, maybe we can release it. If we released a version on the Xbox 360 without Metal Gear Solid 4, then it’s not The Legacy Collection.”
The fourth entry in the franchise was only ever released on the PS3, although it could be played on PS4 via PS Now until recently.
On top of that, Konami announced last year that it was “temporarily” pulling Metal Gear Solid 2 and 3 from all storefronts due to licensing issues around in-game historical footage. Both games are still missing from storefronts as of the publication of this story.