For both Encounter.reasonCode *and *reasonReference, there is a comment that says:

"For systems that need to know which was the primary diagnosis, these will be marked with the standard extension primaryDiagnosis (which is a sequence value rather than a flag, 1 = primary diagnosis)."

I don't see an extension named primaryDiagnosis anymore (I do see it in STU3, but not in R4 or current build).

I assume the comment should just be deleted since Encounter.diagnosis.use and rank provide similar capabilities