With the recent going ons of The War Z and mass Steam reports, it's finally been revoked from Steam. You can still find the store page, but the purchase button is no longer there. Kudos to Valve for listening to the community on this one. Judging by the statement, anyone who has purchased it on Steam will still be allowed to and anyone seeking a refund can create a ticket with Valve. Also, it appears that The War Z may release on Steam again for purchase down the road when it's more stable.
"From time to time a mistake can be made and one was made by prematurely issuing a copy of War Z for sale via Steam. We apologize for this and have temporary removed the sale offering of the title until we have time to work with the developer and have confidence in a new build. Those who purchased the game and wish to continue playing it via Steam may do so. Those who purchased the title via Steam and are unhappy with what they received may seek a refund by creating a ticket at our support site here:https://support.steampowered.com/newticket.php
Again, we apologize any inconvenience." - Valve