In recent years, Appl e greatly increases its update rhythm. This also lost another important option for some users: downgrades. Together with the frequent updates pushes one since then quickly before older software versions. After all, the company requires activation via the Apple servers after every upgrade or downgrade. If a version is no longer supported, activation will be denied and the device will not go beyond the setup screen.
For that reason, it’s little wonder that Apple is not completing iOS 11.3.1 signing two weeks after the release of iOS 11.4, making it impossible to downgrade to an older version of the software.