It became known that the latest version of the Green Robot does not allow the device to roll back to the old versions of the system. And if earlier Google advised device manufacturers to implement protection against rollback, now this condition has become mandatory for all mobile devices that claim to Android 9 Pie.
Google, apparently, is trying to protect itself from kickbacks due to the desire to simplify the procedure for future software updates, as well as to increase the security of devices.
Protection from rollback is performed by the Verified Boot 2.0 service, which prevents the downloading of older firmware to the system. Thus, users who like to change firmware, install Android Pie is not recommended.