My guess is that it's not complete or stable or qualified enough for them to publish it as a supported feature. Maybe the power draw is too high and it trigger

My guess is that it's not complete or stable or qualified enough for them to publish it as a supported feature. Maybe the power draw is too high and it triggers thermal throttling too quickly, similar to early Intel AVX-512? Maybe they are still sorting out how to have each process choose SVE vs NEON mode, so libraries using the other one don't cause a fault? (Note that this seems limited to M4 and newer CPUs.) I agree this is an important feature. I have very often wished that my Mac had wider vectors.