iOS 11 was released by Apple almost a month ago. As usual, Apple this week discontinued the downgrade option for its users. So, whoever downloaded the update, but would like to go back to iOS 10, is no longer possible.
Apple has suspended digital subscriptions for versions 10.3.3 and 11.0, according to 9to5Mac. Therefore, through the traditional method of downgrading, which also includes downloading the firmware and connecting the iPhone or iPad to iTunes on the computer or Mac, it becomes impossible to reinstall such versions of the system.
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The only option for those who really want to go back to the old version of iOS is to perform a jailbreak, which violates the device's warranty, even, if not done well, can cause damage to the firmware.
Apple, so far, even with little time to release iOS 11, has already released two new updates for the recent version, which fix bugs and security flaws of the first version.