I wish I could keep using my current unsupported android phone but slowly all my critical apps are no longer working including the banking apps which won't support my android version. What do you do about that?
Good resource: https://github.com/phhusson/treble_experimentations/wiki
LineageOS is in the same boat as GrapheneOS in this regard.
"GrapheneOS passes the basicIntegrity check but isn't certified by Google so it fails the ctsProfileMatch check. Most apps currently only enforce weak software-based attestation which can be bypassed by spoofing what it checks."