Turns out that the game will not be arriving at all. Warner Bros. has confirmed on Steam that Batman: Arkham Knight will not be making its way onto the Mac or Linux platforms, and that they will now be offering refunds for the game. In their statement, “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.”
It is unclear as to why the Mac and Linux versions were cancelled, but perhaps it could be due to the issues found on the PC version that they did not manage to fix yet for other platforms, or maybe they decided that it would be cheaper to scrap plans for the Mac and Linux version of the game rather than have to support it.
Either way it looks like Mac and Linux gamers are out of luck. Of course, if you run a Mac or a Linux device, you could either run software like Boot Camp, or dual boot your Linux machine with Windows, but for now you can forget about native support.