Rockstar is learning first hand about the pitfalls of trusting your game's functionality to a third-party service. The developer has confirmed to The Verge that it stopped selling Grand Theft Auto 4 on Steam after Microsoft stopped supporting the Games for Windows Live platform the title depends on. It's "no longer possible" to create the extra game keys needed to sell GTA4, a Rockstar spokesperson said. You can still play the Steam version if you already have a copy, but newcomers will have to go somewhere else to experience Niko Bellic's life of crime.
Source: The Verge