As noted by the news release from CalyxOS and Mastodon thread from GrapheneOS, Google did not release the Pixel device-specific source code alongside their Android 16 AOSP release like they usually do. I think many of us, including myself, are hoping this will be published in the near future, but considering they moved AOSP development behind closed doors earlier this year, it's more likely Google has stopped publishing this section or their code altogether, making development of custom ROMs for Pixel devices significantly more difficult. Sad news for the Android ecosystem, and for open source in general.