Shurakai At the moment face unlock is a feature on just one series actively supported by Google (the 8s). There are still many GrapheneOS users on the 6 and 7 series devices, plus Tablet and Fold, and there are many bugs and feature requests that would benefit all of those users in addition to 8 users (as just one example: reliable backups).
All in all, it is possible that GrapheneOS might never support face unlock on the 8 series, or that it might take a genuinely long time. Please note that I am merely discussing plausibly-relevant factors - I do not speak for the GrapheneOS project.