Grand Theft Auto 4’s Steam page no longer lets you buy the game, though there’s no explanation why. GTA Vice City did vanish from the marketplace briefly back in 2012 due to a copyright claim over its inclusion of the song Wanna Be Startin’ Something by Michael Jackson, but Rockstar has been more on the ball with licencing issues since then. As recently as 2018, GTA 4 was patched to replace several songs from Vladivostok FM in advance of their licences expiring.
Another possibility is that it’s related to Games For Windows Live, which GTA 4 still requires. Each game on GFWL needs a unique key generated by Microsoft, and its possible Valve has unexpectedly run out of them or run into an issue obtaining more. With luck, perhaps Rockstar is finally patching GFWL out of GTA 4 altogether.
There’s also the chance that Rockstar is pulling GTA 4 from Steam altogether in advance of making it exclusive to their own Rockstar Launcher.
As always, anyone who has already bought GTA 4 through Steam will keep their copy and be able to download it in the future.