Hello,
I can add Metal backend for VkFFT (https://github.com/DTolm/VkFFT) as it is already abstracted to support different API code generation for FFTs. However, I don't have any Apple machine to test this on currently. There are reports from people who launched the VkFFT Vulkan backend with MoltenVK and OpenCL seems to be still supported by Apple as well.
If Apple engineers are interested in this, feel free to contact me.
Best regards,
Dmitrii