In STU3, MedicationStatement had separate elements for status and taken. Based on the prior guidance, http://hl7.org/fhir/STU3/medicationstatement.html#11.4.3.3, we had the ability to differentiate between:
1) Patient said that the medication was not taken, the provider still wants the patient to take the medication. For example, the provider may want to retain over the counter (OTC) "prenatal vitamins" for a pregnant woman on the active medication list.
2) Patient said that the medication was not taken, and the provider wants to remove the medication from the medication list. For example, the provider doesn't wish to retain an OTC pain medication.

That capability was lost when taken (boolean) was combined into MedicationStatement.status (=not-taken).

The only way we can use MedicationStatement.derivedFrom (MedicationRequest.status) is if all medication statements are within scope of the MedicationRequest – and I didn't think that we could use MedicationRequest for recorded/historical statements of use, although I see that OTC is within scope of MedicationRequest