What iOS version can be downgraded to iOS 10?
No, It is not possible to downgrade a device running iOS 11 to iOS 10.

When iOS is installed and activated on a device, the installation is verified with Apple's servers.

Apple routinely stops verifying older versions of iOS (by stopping signing them) so as to encourage users to remain on the most latest release.

Currently only iOS 11.4.1 is signed by Apple’s server and it isn’t possible to downgrade past it if it’s installed.