Thankfully there is a much more simpler explanation. While it had been initially speculated that the game had been pulled for good, 2K Games later released a statement in which they confirmed that the game had been pulled due to issues with iOS 8.4 and higher. The removal of the game was simply to address compatibility issues before returning it to the iTunes App Store.
We suppose this makes sense. After all no one wants to pay for a buggy game, right? According to the company, “Customers who already own BioShock may continue playing the game with iOS version 8.3 or earlier. 2K regrets the initial customer service error and looks forward to delivering an updated version of BioShock for iOS in the near future.”
Also based on the statement if you are planning on upgrading to iOS 8.4 or even iOS 9, perhaps you should hold off on that update, at least until 2K Games gets the game compatible with newer versions of iOS.