As it happens every time, two, three weeks after launching a final version of iOS, Apple delete the signature for old versions of iOS, that they can no longer be installed on iPhone, iPad or iPod touch. Not the same thing happens with Mac, where an older version of macOS / OS X can be reinstalled at any time.
If you have upgraded from iOS 10.3.3 or from an older version to iOS 11, we have bad news. If you want to return to the older version, there is no way to do so. Shortly after, Apple has deleted the signature from her servers that allowed iOS 10.3.3 to be installed using iTunes.
So you even have the IPSW files of the old operating system, they will not help anymore.
If you currently have iPhone, iPad or iPod an older version of iOS, and you do not want to upgrade to iOS 11, make sure you will not run the update by mistake. Once the update process started, you have no chance to install the old iOS version.
Currently, the latest version of the software is iOS 11.0.2, and the only one downgrade option it is at iOS 11.0.1.
Soon, Apple will launch iOS 11.1, which is currently available in beta through the Apple Beta Software Program.