Batman: Arkham Knight was a decent game, but on PC it was an abominable port. It was so bad that publisher Warner Bros. was compelled to remove it from sale weeks after its release, offering refunds to anyone unfortunate enough to buy in early. Nowadays it runs a lot better, but it's fair to say the whole situation was quite traumatic. As of December it was still being patched up, following its October re-release.
Whether that trauma has anything to do with the Mac and Linux versions of Arkham Knight being canceled, I don't know. The cancellation was announced on Steam today in as blunt a manner as possible.
"We are very sorry to confirm that Batman: Arkham Knight will no longer be coming to Mac and Linux," the post reads. "If you have pre-ordered Batman: Arkham Knight for Mac or Linux, please apply for a refund via Steam."
Despite the game still being a bit rough around the edges on PC, Warner Bros. started selling DLC for it back in December. It's a shame the launch was so poor, because beneath the technical shortcomings there's an okay game, hampered somewhat by annoying Batmobile sequences. In his review, Andy Kelly wrote that it's "an entertaining superhero power fantasy, let down by awful Batmobile combat, a laughable villain, and serious performance issues."
On the topic of bad ports, this recent Durante rundown of the disastrous Tales of Symphonia release is well worth ten minutes.