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.
In what way is Vulkan not portable? It runs on all operating systems (Windows 7+, Linux, Android, and Apple via MoltenVK) and all GPUs (AMD GCN, nVidia Kepler, Intel), and shaders (compute and rendering) are to my knowledge standardized in the portable SPIR-V bytecode.
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.
It's kinda portable, but I've had not-great experiences with MoltenVK - piet-gpu doesn't work on it, for reasons I haven't dug into. It may be practical for some people to write Vulkan-only code.
Vulkan is supported on basically all modern platforms except for Apple operating systems, Apple refuses to support open graphics APIs on their platform and there's nothing anyone can do about it - this isn't a Vulkan problem. Even OpenGL is deprecated and support hasn't been updated for years, and that's basically the most open graphics API in existence.
You basically complain about Vulkan not being portable enough because Apple made their ownTM Vulken-like API instead of actually supporting Vulkan. And some other people made a subset of Vulkan working on top of that.
Why don't you complain about Apple not supporting Vulkan instead?
