This link can be useful as well.
https://forum.xda-developers.com/cro...ackup-t3514236
The TA partition is where the DRM key is stored. When the bootloader is unlocked, the DRM keys are being wiped and therefore are lost forever. With the procedure mentioned above we can safely have a backup of the DRM keys, therefore being able to restore the Xperia to its 'like new' conditions in case of warranty repairs.

It is therefore important to backup the TA partition and DRM keys BEFORE unlocking bootloader first time.

The above procedure is based on an exploit present on Android < 7.0, corrected with the Nougat version. Therefore, if your device is shipped with Nougat pre-installed it takes a bit more time since you need to downgrade the phone first to a previous Android version.
I already did it with mine and I can confirm the procedure works.