OpenCL is sadly stagnant. Vulkan is a good choice but not itself portable. There are frameworks such as wgpu that run compute shaders (among other things) portably across a range of GPU hardware.
WGPU is more portable, since it can use not only Vulkan but also other APIs like OpenGL and Direct3D 11, but Vulkan is already very highly portable for almost everyone with a computer modern enough to run anything related to GPU compute.
Why don't you complain about Apple not supporting Vulkan instead?