It's not even undocumented, but explicitly a documented as a feature those CPUs don't have. Originally it was supposed to be fused off physically, I wonder why it wasn't.
Who did this? Phoronix got AVX-512 working, but was very clear that this was unexpected and had a good chance of going away before release. If anything, this is a better test for your reviewers -- if a reviewer has published benchmarks using these unsupported features without mentioning it, you should keep that in mind when listening to their reviews in the future.
It could be that decision to not support AVX512 was made very late in product development and thus early batches didn't have chicken bit disabled or/and fused it off.
There might be gazillions of reason why this was done.
