Is there any method to disable encryption after A factory reset on Android 6.0?
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For me (Lenovo P2, Android 8.1/LineageOS 15.1) the TWRP procedure below worked although with one side effect. It may also work on Android 6.
Please note that it requires an unlocked bootloader, a custom recovery image (standard for custom ROMs), blanks the /data and internal /sdcard partitions and involves rooting - note that this may result in problems with some security-sensitive programs, such as banking/payment apps.
It is essentially this one for a different device, apart from the steps to mount storage and using a different zip (important); the screenshots are instructive. It is worth being familiar with the recovery program.
I would advise studying/reading up a bit on this - I somehow failed to do /data properly, so had to reinstall and reconfigure all apps.
Side effect on Lenovo P2 is that the fingerprint sensor and settings do not work. (If I encrypt the data partition later from within the Android settings, then it works again.)