Apple recently closed the iOS 11.2.6 system authentication channel. This means that for iPhones that have been upgraded to iOS 11.3, iPad and iPod Touch users will no longer be able to fall back to earlier versions of the system.
At present, Apple has begun to push iOS 11.4 developer preview and public beta to consumers. It is expected that the official version of iOS 11.4 will be officially launched in June. This is also the last big update before iOS 12.