AMX is available in userspace, but only used by their Accelerate.framework library. Apple does not document or expect them to be used by any apps, and I expect they'd reject any App Store submissions that use them, as I doubt they guarantee their continued existence in their current form in future CPUs.

FWIW, the compresssion instructions are used by the kernel, and I don't even know if they work from userspace. I've only ever tried them in EL2.