Q. Does Remote Credential Guard work with Windows 8.1 as the Group Policy specifies Windows 8.1 and above?
A. No, Remote Credential Guard only works for Windows 10 anniversary edition (1607) and later. While the Group Policy Object for Restrict delegation of credentials to remote servers says Windows 8.1 as a minimum this is because the same policy (Computer Configuration - Administrative Templates - System - Credentials Delegation - Restrict delegation of credentials to remote servers) can be used to configure Restricted Admin which does work in Windows 8.1 but the Remote Credential Guard requirements would not work.