It shouldn't come as a shock to anyone that Warner Bros. has announced that the Mac and Linux versions of Batman: Arkham Knight have been canceled, since the regular Windows version suffered not one but two buggy launches, and still has major issues even now.
Warner Bros.in the game's Steam Community forum that "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."
There was little chance Rocksteady was going to be able to add additional platforms for the game when the studio couldn't get the PC version right.
Batman: Arkham Knight was first launched on Steam on 23rd June, 2015, got pulled, then re-released on the 28th October.