Grand Theft Auto IV, a personal favourite of mine and now infamous Cousin Lets Go Bowling-em-up, has mysteriously disappeared from Steam, no longer available for purchase.
There's no word yet on the reason behind this, but there are a few theories - the main one being potential music rights issues. Vice City apparently suffered a similar problem years ago and ended up delisted for some period of time, although Rockstar have patched GTA IV several times to get around this and remove songs, so if this is the reason, it's an oversight on Rockstar's part.
One other theory that is being muted is due to Games for Windows Live, the platform is long abandoned by Microsoft and although it's servers are still running, there may have been some sort of unforeseen change or issue which has caused the game to stop working.
Hopefully Rockstar will eventually get round to patching Games for Windows Live out of the game, that'd be nice. It's a really hard game to get working on Windows 10 I found recently.
If you already have the game in your library, don't worry, it'll still be there, but this means people won't be able to buy the game anymore. We'll be sure to post any updates as we learn them.