Apple is currently signing iOS 10 version updates for many devices including iPhone 5, iPhone 6 and iPhone 7 models as shown by IPSW.me. Whilst the upgrade servers remain in this state, users are able to downgrade from iOS 11 by restoring their firmware to iOS 10.2 and 10.3.
It is not clear why Apple is suddenly allowing this to happen and there is a good chance it is a bug. If you want to downgrade your OS, get in quick before Apple reverts back to signing iOS 11.2 and later only.
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Again, Apple has announced a policy change so at the moment the most likely scenario is that this is unintentional and will be reverted soon, unless the company says otherwise.
iOS 6 and iOS 7 are also being signed according to the IPSW.me tracker although most users attempting to go back that far are hitting hurdles with activation and other issues.
More Info: 9to5mac.com