I'm reading that you can use iTunes revert an iOS device to 'original factory settings', but this is hard to believe. Will it actually allow you to return to the earliest version of the OS that was installed on the device, or will it just give you a fresh install of the latest OS that was installed on the device?
Restoring to factory settings using iTunes installs the latest version of iOS. https://support.apple.com/en-ap/HT201252
In my experience, iTunes just gives you a fresh install of the latest OS. It has to use the same OS that is installed on your device because while reverting back to the old changes - it can not physically look up the information for the iOS that originally came with your device. When you update your device, your iPhone saves over the old iOS settings so there is no way it can revert back to those changes.