In STU3, OperationOutcome.issue.severity was marked 'Is Modifier = true' (http://hl7.org/fhir/stu3/operationoutcome-definitions.html#OperationOutcome.issue.severity).

This was removed in R4, but the comment 'This is labeled as "Is Modifier"...' is unchanged (http://hl7.org/fhir/R4/operationoutcome-definitions.html#OperationOutcome.issue.severity).

Given this field is no longer considered a modifier, the comment should be updated/removed.