Valve confirms temporary removal
The game is still listed as available, but any attempt to find the actual product and purchase will yield nothing.
The removal from Steam is temporary, according to a comment from Valve made to Kotaku, as the company works with Hammerpoint to work on a "new build." Those who bought the game and are displeased with their purchase are invited to open a support ticked with Steam on its support page.
It's been a downward spiral for Hammerpoint ever since War Z's release. Its store page listed a number of false claims, while anybody complaining that they felt cheated had their posts removed from message boards and their accounts banned by overzealous moderators. However, attempts to suppress dissent by the developer proved futile. The outrage of angry online gamers, as always, knows no barrier.
[Thanks to the MANY people who told us about this!]