The problem began on Thursday when Apple (probably by mistake) completely stopped signing installations of all iOS versions for Iphone 4S, Ipad 1, Ipad 2, Ipad 3, and for a bunch of older Ipod Touch models.
After a while, Apple again began signing iOS installations – but this time for all compatible Iphone, Ipad and Ipod models. An Iphone 5 user could therefore have installed iOS 6.1.3 on his mobile, although it should not be possible to downgrade so far back. It writes Macrumors, and points out that, for example, iOS 11.1.2 ended in December.
At Reddit, users describe how to downgrade a tired Iphone 5S to IOS 7 for a short period of time, and how an Iphone 7 user downgraded to iOS 10.3 to jailbreak.
However, the mistake has been made now, and it's no longer possible to try what it used to be in the iOS world.