There is a API restructuring that has to happen here (for instance, in track 2 SDKs receiving deferred messages is treated as a batch operation, without a callback).

Beyond that there are quality issues we need to be aware of:

• (fixed) There's a bug currently in azure-service-bus-go where we can't properly retrieve deferred messages for a dead letter queue: azure-service-bus-go: #241
• Another one, filed by me, ([service-bus] Implement defer in the new track 2 receiver #15210) where you can't defer a message if you're in the 'defer' message handler.