Not sure if I'd use GrapheneOS even if it was available on other devices though. Really not a fan of the hostile attitude towards rooting when it's needed for basic functionality like backups — actual, reliable backups for every app, unlike those provided by the built-in solution.
My OnePlus3 (2017-ish?) can do that.
It's not even a feature, but standard android bootloader will do this much. Vendors deliberately remove such features, if not disable phone unlocking outright[0].