SYCL is probably the most up-to-date CUDA alternative for all intents and purposes, at least if one likes modern C++ style (and lambdas inside lambdas). Expose it as C and get bindings to any other language for relatively little effort as well since it’s just C++. With AdaptiveCpp you can also compile SYCL to CUDA so both ways work with the CUDA dialect (PCUDA).

SYCL, as well as AdaptiveCpp, is a relatively active project though and has been for several years, feeding into the C++ standards committee work and is supported by several large organisations, including US national labs and several European universities. I suppose it’s worth keeping track of for people in related fields.

I suppose it’s just really hard to beat the head start and ecosystem integration NVIDIA has with CUDA.