A statement just posted on the company's forums reads:
Dear Batman: Arkham Knight PC owners,
We want to apologise to those of you who are experiencing performance issues with Batman: Arkham Knight on PC. We take these issues very seriously and have therefore decided to suspend future game sales of the PC version while we work to address these issues to satisfy our quality standards. We greatly value our customers and know that while there are a significant amount of players who are enjoying the game on PC, we want to do whatever we can to make the experience better for PC players overall.
Thank you to those players who have already given valuable feedback. We are continuously monitoring all threads posted in the Official Batman: Arkham Knight Community and Steam forums, as well as any issues logged with our Customer Support (support.wbgames.com). If you purchased your copy of the game and are not satisfied with your experience, then we ask for your patience while these issues are resolved. If desired, you can request a refund at help.steampowered.com (Steam refund policies can be found here: http://store.steampowered.com/steam_refunds) or the retail location where you purchased the game.
The Batman: Arkham fans have continually supported the franchise to its current height of success, and we want to thank you for your patience as we work to deliver an updated version of Batman: Arkham Knight on PC so you can all enjoy the final chapter of the Batman: Arkham series as it was meant to be played.
Wow. Drastic and almost unprecedented, but given the state the game was in, welcome. Note just how severe a move this is: not only are they taking one of the biggest games of the year off the (virtual) shelf, but they're giving you clear instructions on how to get a refund.
Despite Warner's statement, at time of posting we were still able to purchase the game from Steam. The game had disappeared from all promotional places and new release charts, but a search would still bring you to the game's store page.
See? The Season Pass is still there, but the game isn't showing on the New Releases section.
A specific search for the game, though, will still take you to its store page, where it's still available for purchase.