Google removed Pixel support from AOSP with the release of Android 16. They did so by no longer pushing the device trees, kernel drivers, userspace drivers/HALs and Pixel exclusive releases. That means there are no longer device branches pushed to AOSP for the OS releases used by recently released devices or variations from the standard releases. The monthly and QPR1/QPR3 are now considered fully Pixel exclusive and no longer pushed to AOSP either. That's why this code isn't pushed to AOSP. The remaining non-Pixel-exclusive releases of Android are still pushed to AOSP at launch.
Google is abusing their control over Android to give Pixels inherent advantages over other Android devices. However, in doing so they've also harmed the value prospect of Pixels by taking away official AOSP support which has made it much harder to support Pixels for other operating systems and even for app developers.
Pixels are now on Pixel exclusive QPR1 and QPR3 branches for half the year without sources available. For the other half of the year, they're on the same major yearly and QPR2 branches as other devices. Android 17 QPR1 is the latest major release for Pixels but Android 17 is still the latest for non-Pixel devices. Android 17 QPR2 will be the latest for both in December 2026.
They're not pushing the monthly, QPR1 and QPR3 releases to AOSP with a delay but rather not at all since those are only for Pixels and AOSP doesn't support Pixels anymore.