GTA IV is currently unavailable for purchase on Steam right now. Rockstar’s 2008 open-world crime game is still available for download for existing owners, but no one can buy the game at the moment.
There is a reason for that. A Rockstar spokesperson has told The Verge this was due to the Games For Windows Live. This is the now-defunct PC platform exclusive to Windows Vista. One of the fickles of the system is that it needs to generate keys for games to authenticate the copy.
Here is the full statement:
“Grand Theft Auto IV was originally created for the Games For Windows Live platform. With Microsoft no longer supporting Games For Windows Live, it is no longer possible to generate the additional keys needed to continue selling the current version of the game.
“We are looking at other options for distributing GTAIV for PC and will share more information as soon as we can.”
In short: they ran out of keys, with no way of producing more.
Games of this era have subsequently removed the GFWL integration like Batman Arkham Asylum, Bioshock 2, Fallout 3 and Dark Souls. GTA IV seemed to not do so for this long apparently.
Most likely we will see a GFWL-less re-release to Steam and other platforms sometime soon.
GTA IV is also missing from the Rockstar Games Store and Rockstar Games Launcher, with GFWL being more likely the issue here.
Source: The Verge