Here's a pretty good Linus Tech Tips video where he installs stock AOSP on a Pixel phone and goes over how it's virtually unusable. Just like you say, while the Pixel UI may be Google's vision for how the Android platform should work, they've moved to keeping their UI development private just like every other Android vendor. Meanwhile, stock AOSP has basically been left to rot. https://www.youtube.com/watch?v=-hlRB2izres