There are threads here where people have downgraded when Apple 'accidentally' re-opened a window and allowed signings again. I know in general it is not possible.If you're trying to downgrade iOS 11 to iOS 6, it's sadly not possible. They stopped signing iOS 6 long ago.
Well yes the only way to downgrade it would be if Apple signed all the OS's again.There are threads here where people have downgraded when Apple 'accidentally' re-opened a window and allowed signings again. I know in general it is not possible.