From The Apple Wiki
Private Framework
Availabile8.0 – present

NanoRegistry is a framework that handles some of the Apple Watch pairing.

Allow old WatchOS versions

To enable WatchOS 6.X parings for watches that cannot be updated past WatchOS 6.X, NanoRegistry has a special case.

You can allow +[NRPairingCompatibilityVersionInfo systemVersions] to pair this WatchOS version if you override the pref IOS_PAIRING_EOL_MIN_PAIRING_COMPATIBILITY_VERSION_CHIPIDS, which is usually "28674=10,32770=14". For Watch Series 0, the maximum iOS version is 10, Watch Series 1/2, the maximum iOS version is 14. Be aware that while this will allow pairing, behavior still may be a little buggy.