Per this Zulip topic:
https://chat.fhir.org/#narrow/stream/179166-implementers/topic/Can.20.2F.20should.20an.20Extension.2Eurl.20contain.20an.20optional.20version.3F

The language should be changed from SHALL be the canonical URL of a StructureDefinition that defines the extension to SHALL be a reference to the canonical URL of a StructureDefinition that defines the extension to indicate that we are talking about a reference to the canonical URL of the extension definition (StructureDefinition.url) and not the canonical data type.