NetherRealm's 2011 reboot of Mortal Kombat was initially console-exclusive, released a couple of years later on Steam as Mortal Kombat: Komplete Edition with added DLC including Kratos from God of War and Freddy Krueger from the Nightmare on Elm Street movies as playable characters. And now, it's vanished from Steam with no explanation.
Fans have theorized this may be due to international rights to Freddy Krueger reverting from Warner Bros. (Mortal Kombat's publisher) to the estate of Wes Craven (who wrote and directed A Nightmare on Elm Street). There are plenty of other reasons games have been removed from Steam, however, including expired music rights and rereleases and remasterings of the games in question.
It's not the first time a Mortal Kombat game has been abruptly removed from Steam. The Mortal Kombat Arcade Kollection was pulled in 2015 after the death of Games For Windows Live, which it relied on.