Slitherine

I am a steam user who had left steam for 3 years. And after I came back, I found I am very interesed in Sovereignty: Crown of Kings.

But I am so upset that I find I had missed it, why it coun't be bought on Steam or Slitherine?

I don't know where could I buy it now? Does anyone (publisher or developer) still has any steam key for it which could sell to me?
I really like and want it.

I find myself in much the same position, many months later...