Rockstar Games has had to remove Grand Theft Auto IV from sale on Steam due to the discontinued support of Games for Windows Live by Microsoft, which occurred quite a while ago. The game being removed is due to not being able to create new keys for distribution, though those who own the game can still play Grand Theft Auto IV.
The Verge contacted Rockstar Games which said in a 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.”
Grand Theft Auto IV is not available to download at the moment on PC, including not on the Rockstar Launcher. It is likely that the game will pop up on the Rockstar Launcher as well as returning to other stores like Steam. How long it will take will depend on how quickly Rockstar Games can remove the Games for Windows Live link from Grand Theft Auto IV, and make sure the game is running fine.
Source: The Verge