The birth sex extension is inherited from US Core Patient - the only change in the USPHPL profile to this data element is to make it "must support" to align with USCDI requirements (see: http://hl7.org/fhir/us/core/StructureDefinition-us-core-patient.html#mandatory-and-must-support-data-elements - section "Additional USCDI Requirements"). US Core does NOT make the BirthSex extension must support - they removed the must support in a previous release, but add guidance that to align with USCDI one would need to make birthSex must support.