As a member of Android Study Group I found a reply from Chris Banes (Google employee) to someone else asking the same question:

It becomes hard to implement this correctly when the parent recycles views. What happens if only one child wants to handle NS? Lots of edge cases here, and I’m not totally sold on the use cases I’ve heard.

In summary it’s hard to implement properly and not worth the effort.

You can always disable nested scrolling on the child NestedScrollView and use “normal” scrolling.