@paul: Probably the reason why nobody elaborates on this is because the reason seems (at least to them) to be obvious… it is because the stable branches of Android do not include the code necessary for the board to function, or at least to function fully.
While there are some files in stable branches pertaining to the Hikey 960, they are not complete and will not work.
To put it another way, the reason why it won’t work with a stable branch of Android 8.0 is the same reason why it won’t work with a stable branch of Android 1.0.
My guess (and it is only a guess) is that you can anticipate support in a stable “P”.
Having said that, there is no technical reason why you could NOT make it work with a stable Android 8.0, but it won’t be a simple matter of running “lunch hikey960-userdebug; make -j17”, but rather you will have to get your hands really dirty.