I include the Fire tablets in my assessment of AOSP: the app stores have diverged to the point where you increasingly often need different APKs for FireOS and Google Play, and that divergence will continue to grow as Google continues to expand the proprietary Play API surfaces.

Even before Google started moving most API investment out of AOSP and into Google Play directly, FireOS has never felt like a "real" Android. I'd wager most of its tablet users don't even realize the connection to Android. At this point, how much does Amazon really benefit from AOSP that aren't already handling themselves?

This is entirely my own supposition, but based on what I've seen of the entirely different apps platform and ecosystem for the Echo devices, especially the Echo Show "tablet", and how well it is currently doing in the market, it is very easy for me to imagine Amazon will also be getting out of the AOSP game eventually on its Fire tablets/sticks as well. It's easy to imagine Amazon would be happier to have an ecosystem it more directly owns (and doesn't have to keep fighting a cold war with Google Play).

Only the 3rd generation Fire TV Stick runs Fire OS 6, based on Android 7.1. The latest Fire tablet runs Fire OS 5.6, based on Android 5.1:

https://en.wikipedia.org/wiki/Fire_OS#List_of_Fire_OS_versio...