Is there any public documentation as to what these features would be, and has anyone discovered ways to turn them on?

I remember hearing a few years back about Intel rolling a special weird Xeon SKU with VNNI/bfloat16 tacked onto a different core architecture that got cancelled, specifically because Facebook backed out of the project. However, I think that was intended to be a publicly-available SKU that just didn't make monetary sense to produce in volume.

There's also the occasional off-roadmap SKUs that eventually wind up on eBay or wherever... but my impression was that these were just ways for Intel to obfuscate whatever special volume deals they were giving certain hyperscalars.