There are no Canadian exceptions (and can't be). If a country wishes to call their implementation FHIR conformant, it must fully comply with the international specification, no variation permitted. However, countries can define extensions and set constraints on things like what elements and extensions need to be present, what terminologies need to be used, etc. None of that will change any of the base classes. In theory, you could define derived classes that enforced the Canadian constraints, but that's a challenging endeavor and, to my knowledge, not one that many, if any, has done successfully.
Thus far, all Canadian implementation has focused on R4. R5 is still in development and not stable and therefore not really suitable for implementation. Once it comes out in a year or so, I expect there'll be some discussion around when/if to migrate to it.
Terminology is typically retrieved as FHIR resources from a terminology server (
https://tgateway.infoway-inforoute.ca/). I know the Infoway tx server can source CCDD codes. I'm not sure whether it provides DINs as well.
Lloyd