I always thought the iMac pro was a stop gap machine until the Mac Pro came out, so it's very possible they will retire it from the line up.
It's been 774 days since the iMac pro was released, I find it hard to believe Apple couldn't have refreshed it during that time which lends weight to the idea that it's on it's way out.
https://buyersguide.macrumors.com/#iMac_Pro