How to acquire a Android phone with locked bootloader?

As we know that some devices come with locked bootloaders like Sony, HUAWEI, HTC ... If you try to unlock bootloader, the data would disappear!!!

Take the HTC d826y for example and our goal are to does physical acquisition so as to carve deleted data. But HTC d826y isn't on the support list of ufed or XRY and so there are nothing they can do. If you want to the root this phone, you have to unlock bootloader first. Some may say backup all data first and then root it, and how to backup "All" data was still a problem for an unrooted Android p Hone.

That's a very serious problem when bumping to some phone with locked bootloaders~

How to acquire a Android phone with locked bootloader?