The point of the trash can Mac Pro, was to use OpenCL to leverage two mid level GPUs as a cheaper and higher performance alternative to one high end and expensive GPU.
Developers never really adopted OpenCL, and the whole idea was dropped.
Plus Khronos, as usual, was too C focus while Apple wanted to go into another direction.
Note how all proprietary shading languages are actually C++ dialects.
Only after loosing to the multiple language CUDA eco-system did Khronos start discussing about supporting C++, and even today it isn't as if OEMs are racing to bring SYCL drivers into the market.
The point of the trash can Mac Pro, was to use OpenCL to leverage two mid level GPUs as a cheaper and higher performance alternative to one high end and expensive GPU.
Developers never really adopted OpenCL, and the whole idea was dropped.