Downgrade via initial/factory firmware was blocked starting with fw 5.8.8. Downgrade via any older official/consumer bin file was blocked sometime around fw 5.4 or 5.5, IIRC. Paperwhite 1st gen is the only exception (it can be downgraded from fw 5.6.1.1 via "reboot before disconnecting from PC" method) to that rule.
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