Batman: Arkham Knight for Linux and OS X is now no more. A Batman: Arkham Knight developer at Warner Bros announced this afternoon on SteamCommunity.com that the port for OS X and Linux has been canceled.
This port was supposed to have been done by Feral Interactive and was delayed to this spring while the Linux/SteamOS initial confirmation of the port happening was about a year ago.
Batman: Arkham Knight was released for Windows in June of 2015 and powered by Unreal Engine 3. However, this game was heavily criticized for performance issues and other technical problems on Windows. Given the range of issues encountered that they even had to suspend sales for a period of time, it's not too surprising that Warner Bros has now decided to cancel this Batman game for OS X and Linux. If you bought it for OS X or Linux, the Warner Bros developer recommends applying for a refund via Steam.