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One approach is for the controller to allow unregistering controllers non-durably if the metadata version does not support the UnregisterControllerRecord. This gives operators a path to migrate existing clusters onto a MV that supports this record, where they can persist the unregister record. One issue with this approach is that the UX is not good, requiring the user to unregister the controller twice.

Another approach is to allow the active controller to unregister observer controllers that the user specifies as part of a MV upgrade to support UnregisterControllerRecord. The main issue with this approach is that it makes the UX for kafka-features upgrade pretty complex, and it may be overkill in non-managed deployments, where operators can simply provision a controller to update its stale registration.

Test Plan

Add an integration test for unregistering a controller in both static quorum and dynamic quorum clusters. 

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