Grand Theft Auto IV, Rockstar Games' 2008 entry in the acclaimed Grand Theft Auto series is no longer available to purchase on the Steam store, neither is the Complete Edition of the game. Strangely, the standalone title: Episodes from Liberty City is still up for sale if bought outside of the Complete Edition.
Rockstar has given no statement on the reason for this removal. Historically, the Grand Theft Auto series has dealt with numerous issues regarding in-game music licensing, including one instance in 2018 which saw GTA IV patched and some music tracks removed. It's possible that this de-listing has occurred as a result of additional licensing concerns, in which case it's possible that the game may return to Steam in due course.
Episodes from Liberty City being spared does raise some questions however: the standalone content not only has the original game's entire music library, it actually expands on it. If any piece of GTA IV content was going to cause music licensing issues, it's Episodes from Liberty City. For now, we can only wait for an official statement from Rockstar.