Per https://chat.fhir.org/#narrow/stream/179252-IG-creation/topic/Do.20we.20have.20to.20define.20property.20codes.3F, it appears that the intent is that 'code' properties allow pointing to codes defined in the code system, while 'string' properties allow specifying arbitrary strings.  However, for some properties, it's desirable to constrain the allowed values for a property without having to define those allowed values as valid 'codes' in the code system.

Suggest defining a new extension called CodeSystem-property-allowed-value of type 0..* [Coding | string | integer] with a context of CodeSystem.property

Short: "Allowed value for this property"

Definition: "If this extension is present, then instances of this property may only have one of the values indicated by repetitions of this extension"

Comment: "The type of the value of this extension must be the same as the 'type' of the property"