While I’d like to believe that the recent removal of Aliens vs. Predator and Aliens: Colonial Marines — two near-universally maligned games that are perhaps best left forgotten — from the Steam store was due to the fact that they’re both awful and completely unworthy of anyone’s time, my guess is Sega just needs to renew their license.
Whatever the reason is behind their delisting, it hasn’t affected Alien: Isolation or Aliens vs. Predator Classic 2000, and since those are the only two Aliens games that are worth playing, I recommend you look there to get your bug-squashing fix.
I was able to confirm that both games are still available on other platforms, including PS3 and Xbox 360, but seriously, don’t spend your money on them.