Google's December update for its Pixel family of smartphone is now rolling out to the Pixel 3, 3 XL, 3a, 3a XL, Pixel 2, and 2 XL. In the support thread containing the changelogs, it's mentioned that the Google Pixel 4 and 4 XL will begin receiving the December update next week.
The excerpt at the end of the following quote mentions improvements to face unlock, so we might expect to see the Pixel 4 and 4 XL face unlock updated with improved security.
In addition to the specific items listed above, the December update includes hundreds of various fixes and improvements across many areas including performance, stability, camera, display, and face unlock.
Otherwise, here are some of the fixes mentioned for the December release: