Batman Arkham Knight is no longer in development for Mac and Linux. Feral Interactive was working on the ports for Mac Linux and SteamOS, but the projects have been cancelled.
In a post on Steam community forum:
We are very sorry to confirm that Batman: Arkham Knight will no longer be coming to Mac and Linux. If you have pre-ordered Batman: Arkham Knight for Mac or Linux, please apply for a refund via Steam.
Feral ported Batman: Arkham Asylum and Arkham City to Mac and did a decent job. They took the job again of porting Arkham Knight but it looks like they couldn’t handle. The game was supposed to release late last year but was delayed on the same day as its PC version was put back on sale.
Batman Arkham Knight was scheduled for Spring 2016 on Linux and Mac.
Batman Arkham series without a doubt gave us the best Dark Knight games ever. However, seeing it end like this is a little sad. Even though the game itself was great, its PC port was a complete disaster. On top of that, cancelling Mac and Linux ports is just like rubbing salt on our wounds.
Console versions were handled by Rocksteady who released a playable and optimized game. Sadly, PC, Mac and Linux ports were outsourced to different studios. Rocksteady did try to help out once they were done with the console version, but the issues just kept popping up and as a result, the final entry of Batman Arkham series will forever be remembered as a train wreck.