Forwards compatibility of DSTU2 (draft standard for trial use) was never intended. That is the short answer.
There are breaking changes between DSTU2 and R4. Even to the extent that some FHIR Resources in DSTU2 no longer exist and names were changed or collapsed into a single Resource (ProcedureRequest + ReferralRequest => ServiceRequest). DSTU2 => SDA => R4 is a path, but at some point we stopped supporting DSTU2. There would be need to analyze data loss and integrity of the R4 data if it is still possible.
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The FHIR subscription architecture will change in FHIR R5. The SubscriptionTopic Resource exists in FHIR R 5.0.0 snapshot but did not exist in R4. Therefore it has maturity level 0. Advancement of maturity level is a prescribed processs which requires use in connectathons as well as in production (once to level 3). FHIR Subscription Resource is maturity level 2. Maturity level does not relate so much to whether a Resource is implementable as it does to whether the Resource is likely to change.