FHIR data requirements provide guidance on what information is valuable, and even what may be required. These requirements should enhance the quality of data collected by conformant systems.

But these systems don't just collect data; they also preserve legacy data and receive data from other systems. That data may not conform to the best practice expectations specified in the standards.

If a system has data about a patient that does not conform to FHIR requirements - e.g., is missing a required field - FHIR supports the "null" approach using the Data Absent Reason extension.

The "required" terminology binding, however, requires a proper value, prohibiting the use of this extension.

The US Core documents 5 properties that have core required bindings, but have no appropriate “unknown” concept code. (There may be others.)

AllergyIntolerance.clinicalStatus
Condition.clinicalStatus
DocumentReference.status
Immunization.status
Goal.lifecycleStatus

For these properties, nonconformant legacy or partner data cannot be provided in FHIR. This data may be clinically important, or even critical. A "404" page is not appropriate.

There needs to be a way to opt out of these fields - whether by prohibiting Required bindings to value sets without UNK and OTH values (and updating those that exist) or by relaxing the Required guidance.