Arkham Knight is coming back to PC - End of Oct.
Months ago we had to suspend the sales on Arkham Knight. It seems the game will be back very soon.
From Warner's announcement:
As many of you know, we released a major patch for Batman: Arkham Knight PC a few weeks ago. While there were significant performance improvements made to the game, the teams are continuing to work on the additional updates that were outlined in our previous post. We expect these updates to be ready at the end of October, at which time the PC version will be made available for purchase. In addition, support for all DLC that has been released for the console versions will be in place.