Did you buy Prey on Steam? I don't mean the 2017 version from Arkane and Bethedsa, I mean the 2006 version from Human Head Studios and 2K Games. If you didn't and want to now, you can't—the original Prey was quietly removed from Steam a few years ago. If you own it, you should still be able to play it, and if you can find a physical copy you may even be able to register the CD code with Steam. But you can't buy it on Steam anymore.
Prey isn't alone: games vanish from Steam all the time. Sometimes they reappear later, but sometimes they're just plain gone—and this can happen for all sorts of different reasons.
Expired music rights
You can currently buy DLC for Remedy's Alan Wake on Steam—but not the game itself. The reason has to do with music. It's common for a game developer to pay for the rights to songs for a certain period of time—seven years, in the case of Alan Wake—and when that time expires, the game can no longer be sold if it still contains those songs. Grid, Dirt 3, F1 2013, Tony Hawk Hawk Pro Skater HD, and numerous other games have vanished from Steam and other digital marketplaces due to music licenses expiring.
This isn't unique to gaming—we've seen it happen to TV shows like The Wonder Years and WKRP in Cincinati, which can't be re-broadcast or sold once the rights to the music used on the shows have expired. The music either has to be removed from the show and replaced with something else, or new deals with license holders for each song need to be made. Years after a show or game has wrapped production, it's extremely difficult for either of those things to happen.
The same situation has occurred with other games, too: Grand Theft Auto: Vice City was pulled briefly off Steam and other digital marketplaces in 2012, only returning after several songs had been removed. On the plus side, if you already own these games prior to them leaving Steam, you can still play them and enjoy the music.
Reviews causing a stir is nothing new, but it's entirely another matter when a developer tries to boost its Steam review score and word of mouth with fake reviews for its own game. We've seen it a few times now: Acram Digital's board game adaptations were pulled from Steam by Valve after it came to light that a staff member of Acram had been posting positive reviews of their game using different Steam accounts.
Something similar happened in February, when publisher Insel Games attempted to 'encourage' employees to post positive reviews of its game Wild Buster. When Valve got wind of it, they pulled the game from Steam.
House Party, an Early Access game from 2017, quickly rose to popularity before it was suddenly yanked from the Steam store. That was due to complaints of 'pornography' in the game being sent to Valve, though that's as vague and hard to define as the term 'sexual content.' House Party returned a few days later having added a 'censor bar' to certain scenes, though cleverly provided a patch players could use to remove it if they wanted the game to remain uncensored.
House Party isn't the only game targeted with complaints about 'pornography'—several anime-style games, such as HuniePot and the visual novel Mutiny!! were recently warned that they're in violation of Steam guidelines and given deadlines to alter or censor their games, or face being taken off Steam. The tricky thing is that these guidelines are confusing and inconsistent, and it's not clear why these specific games are the subject of complaints, while others that feature sex or nudity aren't.
It's been replaced by a remastered version
Dark Souls: Prepare to Die edition will be leaving Steam on May 25, when Dark Souls: Remastered Edition arrives. As with a few other examples above, this doesn't mean it'll vanish from your Steam library if you own it, but you'll be unable to purchase the older version once the remaster arrives.
This is almost true of The Elder Scrolls V: Skyrim. The original version of Skyrim won't come up in a search of Steam, just the other versions (VR and the Special Edition). It is, however, still there. It appears Bethesda is just hiding it, hoping newcomers will buy the Special Edition instead.
Batman: Arkham Knight is maybe the most well-known example: after launching on PC, the port was widely criticized by due to performance issues, after which publisher Warner Bros. suspended digital sales.
Five Nights at Freddy's World creator Scott Cawthon withdrew his own game from Steam as well, stating he wasn't satisfied with the ratings and reviews it was receiving and apologizing for its state at the time of release. He even offered refunds to Steam users, no matter when it had been purchased or for how long it had been played.
In September of 2017, Valve removed 173 games from Steam, all from the same single developer.
These games appeared to simply have been added in order to mine Steam trading cards for profit. "They generate many thousands of [Steam] keys and hand them out to bots running Steam accounts, which then idle away in their games to collect Trading Cards." A clever ruse, but one Valve is now wise to.
The game isn't good
Lots of games aren't good, but they usually stick around anyway. Not all, however. Afro Samurai 2 was taken off Steam by its makers, and future episodes canceled, because "The game was a failure," according to general manager of developer Versus Evil Steve Escalante.
"If you look at the reviews, it wasn’t that the game was broken or buggy, people just didn’t like it,” he said. Fair enough.
The game wasn't finished
The first Early Access game I ever bought was Under The Ocean, a survival game, way back in 2012. I know Early Access is a gamble, but I enjoyed the game and was willing to be patient as it slowly wound its way through development. Until, eventually, it didn't. Issues arose between members of the team, the money ran out, and the developer removed the game from Steam. He is still, however, working to complete it, judging from recent posts on the game's community page.
The developer threatened to kill Gabe Newell
This is the first and only example, at least I hope, but worth noting to round out the list. As a general rule: don't ever threaten to kill anyone, ever. And not just because it may get your game removed from Steam. Because it's, like, a criminal act and a shitty thing to do.
Paranautical Activity was taken off Steam in October of 2014 after a member of the development team made death threats to Gabe Newell. The developer responsible resigned, then rejoined the studio, then sold the rights to another distribution company, and the game re-arrived on Steam.
They ran out of digital copies?
I began this article with 2006's Prey, and I hoped by the time I finished I'd have found the reason Prey is no longer on Steam. It's not entirely clear, though. In 2009, Kotaku reported that Prey had sold out during a Steam sale, which is a weird thing to happen, all things considered. I've seen some speculation that Steam codes were tied to physical CD-ROM keys provided by publisher 2K, and that's what ran out, though I'd expect more could be generated without much fuss. At any rate, it's now gone, apparently forever. If I find out more, I'll update you.