It's about the Pixel kernel drivers and build system. The source code for the base kernel tree itself is still part of AOSP. Everything related to Pixels is no longer being pushed to AOSP.

They stopped pushing tags for any of the Pixel kernel or userspace driver repositories to AOSP. They also stopped pushing AOSP releases specific to Pixels which is why AOSP now only gets yearly releases, QPR2 releases and security backports to both of those. Other OEMs are meant to use the yearly and QPR2 releases along with the security backports to those so that's all they push. The monthly and QPR1/QPR3 releases aren't used by their OEM partners so they stopped pushing them to AOSP. Those no longer being pushed is because of them deciding AOSP doesn't support Pixels anymore.