Platinum Games' Teenage Mutant Ninja Turtles: Mutants in Manhattan is no longer available to download via Steam, PlayStation Network or Xbox Live, as noted by various sources.
Mutants in Manhattan was released less than a year ago, in May 2016, which makes this recent development all the more surprising. The Amazing Spider-Man and its sequel have been removed from digital stores too, so it might have something to do with licensing issues.
This is not the first time Activision has had an issue like this. Of course you can still pick a physical copy from a regular store, if you wish to play these games. Were you planning on downloading it?