A new bootloader unlocking for older ZTE devices was dicovered!
ZTE does not want to offer bootloader unlocking on their devices, not even for their flagship devices. It's sad, because their devices have a lot of potential. Instead, they focus on making a bunch of really cheap phones, one after another.
How to install stock frimware on the Rogers ZTE Avid Plus Z828
Warning: This Frimware is only for the Canadian Fido/Rogers ZTE Avid Plus. Installing it on the US variants might lead to a bricked device and might create other unexpected problems.