Nikon is characteristically reticent to spell out deeper details of the new AF in the D6. In their video, based on demos of pingpong at the UK Launch, GoW mentioned the ability to delimit the AF zone as well as the 17 options to configure sensors in Group Mode.
https://www.youtube.com/watch?v=sv__lY-TMz4If I recall correctly, the D4 "only" used the D3 AF, and this was tweaked in the D4s - added improvements. 2 generations after the D3, the D5 was the first camera to get a completely new AF System.: cloned with distinct changes into the D500 and D850. Today we read/hear the D6 again gets a completely new AF System above and beyond the D5. This is only one generation later. Nevertheless, the Nikon bashing is strident?! Maybe cynical, but I suspect most of the bleaters will never buy a D6 (neither a Used D4 etc) and quite a few only have a cellphone ;-)
The AF alone is a great leap forward. It will be with us for the next 4 years if not longer. Nikon won't dump all the R&D they've invested, and we can look forward to the algorithms being built into the MILC AF of future Z cameras.
Currently, this site seems to have the best coverage:
https://www.nikon-image.com/products/slr/lineup/d6/features01.html