We came up with the concept for this 2-factor fingerprint unlock feature in 2015 and filed it in the public issue tracker in 2016. This was extremely difficult to implement correctly and we needed to fix multiple upstream Android bugs. The lockscreen will be more robust even if you don't use this.
Discussion
This is now one of the flagship features of GrapheneOS alongside hardened_malloc, hardware memory tagging, hardware-level disabling of the USB-C port, Storage Scopes, Contact Scopes, sandboxed Google Play compatibility layer, etc. It will be harder to port to new versions than our existing features.
Could it being harder to port to new versions than other existing features mean it might be dropped again?