What's better for privacy, an old but inexpensive smartphone running Android 11, or the same smartphone running an up-to-date third-party rebuild of Android 16 or newer
I see your point, but it would be very misleading, since the phone would still have a lot of known holes. Only the OS would get updated, typically not the drivers, driver firmware, possibly not the kernel. The phone would still be easily compromised through all the known RCEs. So you tie up non-profit projects in a lot of extra work to get an improvement that does not really matter.
This is a mess created by the OEMs and they will continue to create this mess until people will stop buying from OEMs that only give lip service to security updates (roll out Android Security Bulletins to show a high patch level, while in reality the phone the phone has many known CVEs).
Android Security Bulletins do list a tiny subset of firmware, Linux kernel, driver and HAL patches so they do require at least very minimal updates to those. Most non-Google-certified operating systems are setting an inaccurate Android security patch level by ignoring the non-AOSP portion of the patches. GrapheneOS doesn't do that but most of the other AOSP-based projects not being certified by Google are doing it. OEMs were caught doing it too but it's not clear if it was intentional in most cases as it is with the alternate operating systems.
Android Security Bulletins set a very low bar since the AOSP patches are available to ship by OEMs 2-6 months prior to the bulletin being published. It's also only High/Critical severity patches being listed. Due to a recent policy change, it's also officially only a subset of the patches for AOSP. That's visible through the Android platform components having patches in the Pixel Update Bulletin for September 2026 despite those being applicable to other operating systems. It's because they no longer want to backport all High and Critical severity patches due to the high volume of issues discovered by AI models. It's similar to how they stopped backporting any Low and Moderate severity patches years ago due to high volume.