To clarify here, it requires both the E-cores disabled and a specific option enabled in the BIOS. The option was removed from the Intel firmware provided to vendors, but all the motherboard vendors found what bit to flip to get it to work. MSI played safe and initial BIOSes didn't have the option, the rest put it in there from launch. MSI are now releasing BIOSes with the AVX-512 option.

Beyond that, AVX-512 is not POR. It's not validated, checked for IEEE accuracy, and YMMV on whether it works at what frequency with the correct outputs.

Source: AnandTech.... where I wrote about it :)