Google flip-flopped on the face unlock/fingerprint reader debate, but seemingly settled on having a physical fingerprint reader. Then Pixel 6 and 6 Pro leaks promised the return of face unlocking, only for the official unveiling to make no mention of it. That may have been a last minute change.
XDA Recognized Developer Freak07 spotted some relevant source code in the Pixel 6 series kernel. Back in July there was a patch that reduced power usage during face unlocking, a feature code named “Tuscany”.
The patch reduced the time the CPU cores spend at a high frequency and only really pushed them during the recognition stage. It looks like the TPU inside the Tensor chipset was going to do much of the heavy lifting (that is its unique feature).
For whatever reason, face unlock is not available on the Pixel 6 duo. Maybe work on the feature didn’t complete in time for launch and it will be finished and delivered to user in a future Pixel feature drop. Maybe.