GrapheneBoy Possibly of interest: https://discuss.grapheneos.org/d/15613-what-features-will-be-missing-in-gos-android-15-version/29
My sense is that there isn't an open-source (e.g., AOSP) implementation of face unlock. It is possible that the GrapheneOS developers could set up enough compatibility code to run Google's face-unlock code (as they have done for Play and for Android Auto and for the thermometer app). But it might take a fair amount of work just to figure out how much work would be needed for a reliable implementation.
If it would be a lot of work, and if the security wouldn't be great, and if it would benefit only people with Pro devices, I can imagine that the developers might decide it doesn't rank as high as things that would genuinely improve security for all GrapheneOS users. The situation with the thermometer was apparently not a lot of work, with no security concerns.
Please note that I don't speak for the GrapheneOS project.