I wanted to get the game but it is no longer available for sale on Steam (yet key holders can still download).

Does anyone have a key they'd share or trade with me?

Does anyone even know why it was removed in the first place?

This seems out of the blue. I'm not sure it was ever on steam... was it? I played it a long time back and still sing the game's praises. I got it off of some other website that I can no longer remember. I had a crappier computer though and never finished it because it kept crashing near the end of the game. I tried to buy it again a year or so ago and it seems to have evaporated into the ether.

Cryostasis was on Steam back in 2009 (a whole 2 years after it was originally released) and was subsequently removed because the publisher for the Steam release lost the IP and they weren't able to sell it anymore. You can probably find traders with gift copies, but otherwise you're probably going to have to buy a retail version which isn't really all that expensive.