The Aliens vs. Predator Steam forum offers a confused post which notes the game associated with the forum is no longer being sold on Steam, though it remains playable for those who have already purchased it. There is an extensive discussion of this and the concerns it raises on reddit. There's also a related post on Polygon noting that a Aliens: Colonial Marines is similarly no longer for sale on Steam, with this forum post raising questions about what's going on which have so far gone unanswered. They also note that for those still interested, Steam codes for Aliens: Colonial Marines can still be found on Amazon along with the Aliens: Colonial Marines Season Pass.