> it’s a much nicer ISA than anything before it (in x86 land).

It's coming from Intel Knights Landing. Previous massively multicore Intel offerings used a Pentium core and had a different 512 bit SIMD instruction set, KL used a Silvermont Atom core and introduced AVX-512 and parts of it were implemented in the Skylake Purley platform in a desperate attempt to give KL more software. With little to no actual adaption and the desperate situation of being stuck on 14nm and overselling those capabilities because they expected most CPUs to move over to 10nm it was no surprise the Knights... chips got the axe. But, the insanity of AVX-512 having an entire menu of possible instruction subsets stayed.