Apple bet on OpenCL more so than GPU compute. GCN Gen 1 was fine for circa 2013 version of OpenCL (version 1.1 ramping up on 1.2). What has been largely AWOL is Apple putting substantive effort into OpenCL 2.0 (and up).
There is nothing past 1.2 here:
https://support.apple.com/en-us/HT202823 "Mac computers that use OpenCL and OpenGL graphics"
That list is calcified on OpenGL support too. Apple changed the bet from OpenCL to Metal and that has had relatively minimal impact on the Mac Pro. Metal isn't a substitute for OpenCL ( there is some overlap in relatively narrow use cases, but generally not equivalent. ). If Apple has stuck OpenCL in a v1.2 deep freeze than Apple has pretty much walked away from what they bet on.