A new warning message on the Pixel Factory Image page says that the Pixel 6, 6 Pro, and 6a can never go back to older versions of Android once they update:.
Previously, this feature would just show a warning message on boot (and it looks like that will still happen on the Pixel 5 and lower), but now, Google plainly says of the Pixel 6, "You will not be able to flash older Android 12 builds.".
If you don't count Android 12L, this is the Pixel 6's first major OS update.
This isn't a big deal for most consumers, but in previous Android versions, it was nice to have an escape hatch if Google came out with a particularly buggy first release.