https://build.fhir.org/branches/R4B/versioning#mp-version (and https://build.fhir.org/versioning.html#mp-version ) indicate that a "naked resource" can claim conformance to a particular version of the base specification by adding a value to the Resource.meta.profile array like this: "http://hl7.org/fhir/3.0/StructureDefinition/Patient"

However, this guidance feels strange to me now that Resource.meta.profile is of type canonical.
The data type definition for canonical at https://build.fhir.org/branches/R4B/datatypes.html#canonical states that this type is "A URI that refers to a resource by its canonical URL (resources with a url property)."

In this case, the canonical references a StructureDefinition that is defined in the core specification.
For example, the StructureDefinition for Patient in R4 is at http://hl7.org/fhir/4.0/StructureDefinition/Patient

However, this StructureDefinition does have a "url" property and that property has an actual value of "http://hl7.org/fhir/StructureDefinition/Patient", not "http://hl7.org/fhir/4.0/StructureDefinition/Patient".

I think that references to the core spec's StructureDefinitions should behave like references to any other StructureDefinition..they should use the StructureDefinition's canonical url.