GTA IV‘s steam page had a sudden change recently where the game is not available for purchase anymore. It was pretty sudden and no official reason was given, but some educated guesses could be made for why. Firstly, GTA IV made use of Microsoft’s doomed Games for Windows LIVE service. Codes have to be generated by Microsoft and it’s possible that they might have run out of keys for the game and can’t sell anymore.
Another possibility which holds a bit more water is that Rockstar might be planning to release the game on its own launcher with Games for Windows LIVE cut out entirely. It would make the most business sense since GTA IV is still one of the few Rockstar games not available on the launcher. The whole GFWL integration is a cumbersome affair for both gamers and the publisher and it would be easier to just remove all the old versions and have a brand new polished version available on the native launcher.
People who have bought GTA IV in the past can still download and play the game as normal, but no additional purchases can be made at this time. There’s a small part of me that thinks they might be planning some kind of remaster for the game, but that’s some pie in the sky dreams. We’ll probably have official word from Rockstar at some point in the near future.