Dec 5, 2019
http://hiddenfly.com/a-step-by-step-guide-to-lock-the-bootloader-on-android/
If a device is only rooted, you can go back to the default version of a phone. In this situation, unrooting can be easy. The SuperSU app can make unrooting easy by removing roots and replace stock recovery of Android. A phone with a custom ROM or Xposed Framework may need alteration of individual parts in the system.
http://hiddenfly.com/a-step-by-step-guide-to-lock-the-bootloader-on-android/