Several possible reasons for this.
Assuming a pessimistic turn of events, Rockstar may have decided to end the sale on Steam and transfer the title to the Rockstar Launcher exclusive platform. This game could bring new users applications, but would also cause great outrage among fans of the series. Does this step make sense? I do not know.
The second theory is that everything is related to Games For Windows Live, which Grand Theft Auto IV, unfortunately, still requires. Each game at GFWL requires a unique key, which is generated by Microsoft. In theory, the key pool may have run out and Valve for some reason has not received any more. However, this seems less likely than that Rockstar can just remove GFWL from its production and is preparing an appropriate patch. After its release, the game could appear in the store.
In a similar way, GTA: Vice City disappeared from Steam in 2012. Short-term absence was then caused by problems with the license for the song Wanna Be Startin 'Something by Michael Jackson. The game returned to the Valve platform when Rockstar removed the controversial song.
If you have Grand Theft Auto IV in your library of games, don't worry – it still works correctly.