GTA 4 cannot be purchased on Steam anymore

Firstly, the reason behind GTA 4 removal could be that Valve might not have enough keys left for players. Secondly, Rockstar might try to move their game from Steam to Rockstar Launcher. Thirdly, it might be due to Music Licences getting expired, and Rockstar would search for replacements before getting a claim. Additionally, GTA 4 Port was very bad, and they might try to make some improvements, hence a major update coming who knows. The last and very least guess could be that Rockstar is now trying to port the game to Steam Multiplayer.