Thanks to an eagle-eyed reader, the publisher Aspyr Media commented on reddit with this:
observer_ is ABSOLUTELY coming to Linux! We had a date we needed to hit, and unfortunately couldnt get linux done within that timeframe. We are working hard towards a late Oct date, but definitely this year.
It's a shame the Linux version didn't make the original cut, but it's pleasing to know Aspyr Media are still pushing forward with Linux versions of games they publish.
Observer does look incredible and it has been reviewed very well on Steam and plenty of other sites have given it their blessing too.
About the game:
What would you do if your fears were hacked? >observer_ is a cyberpunk horror game from Bloober Team, the creators of Layers of Fear. Discover a dark cyberpunk world beset by plagues, war and squalor. Play as the new front line of neural police as you hack into the jagged minds of the insane.