As DSOGaming (opens in new tab) pointed out this morning, Mortal Kombat is about as confirmed for Steam as it can get without an actual announcement. Not only did it get a Steam App ID and achievement list (opens in new tab) a while back, the test app is now being played.
ValveTestApp230800, aka Mortal Kombat, saw a spike in active players (opens in new tab) today after an update, which we have to assume means ValveTestApp230800 is now being tested by ValveTestApp230800 testers. Warner Bros. hasn't officially confirmed a PC version of Mortal Kombat, so until then, we look forward to the release of ValveTestApp230800.
There is the off chance this Steam data was fabricated to divert our attention from some great and mysterious plot taking place within the highest ranks of the PC gaming elite, but devious machinations seem a smidgen less likely than seeing Mortal Kombat on Steam. Occam's razor—the principle that the hypothesis which makes the least assumptions should be tested first—applies here. If not, we'll likely be silenced by Occam's razor wire, the favorite brand of the world's leading hypothetical assassin squads.