Sure, but the signed firmware could be written to delete any stored keys when it accepts an update and the phone's passphrase has not been provided. That's assuming that it manages it's own update process, has the firmware securely stored within it's own die, etc. It's entirely possible ... but only Apple really knows.
I would not actually be shocked if they originally did wipe out stored info on firmware update, but had some issues with people updating their phone and losing everything, so they ifdef'd that particular bit out in the name of usability.
I would not actually be shocked if they originally did wipe out stored info on firmware update, but had some issues with people updating their phone and losing everything, so they ifdef'd that particular bit out in the name of usability.