Here are stock Mate 9 insecure boot images that are modified to not force encryption. You can flash these in fastboot or TWRP, just select Image in TWRP then flash. An insecure boot image lets you use adb remount, and adb push or adb pull etc. to /system and its contents. Basically it's using ADB as root. I'll modify more upon request provided you link me to the boot you want modified.
ALL VERSIONS:
*To remove encryption after flashing to stock , boot into TWRP, format data, reboot recovery then flash a decrypted boot.img. If you're running a stock modified kernel, formatting data alone will not remove encryption. The idea is to always be decrypted. Never boot the system with a stock unmodified boot.img or it will encrypt your internal storage. So if you're fastboot flashing back to stock from another rom, flash a modified boot.img instead of the stock one*
ALL VERSIONS:
*To remove encryption after flashing to stock , boot into TWRP, format data, reboot recovery then flash a decrypted boot.img. If you're running a stock modified kernel, formatting data alone will not remove encryption. The idea is to always be decrypted. Never boot the system with a stock unmodified boot.img or it will encrypt your internal storage. So if you're fastboot flashing back to stock from another rom, flash a modified boot.img instead of the stock one*
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