If some custom behaviour really is needed (though it seems not), a custom operation would be more appropriate, using the dollar prefix e.g. GET /$PatientFlags
Is anyone from NHS England able to provide feedback on this?
It would be really useful to understand whether the strategy of implementing pseudo-resources such as PatientFlag is going to be pursued. This has significant implications for how implementers work with national APIs as they may not be able to use standard FHIR packages.