The subjectCanonical choice was added to ActivityDefinition in R4B. According to the definition of subject[x]:

Canonical references are allowed to support the definition of protocols for drug and substance quality specifications, and is allowed to reference a MedicinalProductDefinition, SubstanceDefinition, AdministrableProductDefinition, ManufacturedItemDefinition, or PackagedProductDefinition resource.

The formal specification for subject[x], however, allows for the canonical to reference anything. Assuming the definition is correct, this should be constrained to the specific types noted in the definition above (and as done in PlanDefinition) – or if the definition is incorrect, it should be fixed and the canonical should still be constrained to whatever the appropriate target types are.