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By default, the new setting will be set to "required"  which means we'll use IncrementalAlterConfigs if possible, and fallback to AlterConfigs API if incompatible. This means there will be a behaviour change while syncing topic configurations in MM2 after this KIP. For example, if changing retentions.ms of a topic in the source cluster, filtered configurations such as follower.replication.throttled.replicas of the topic currently get cleared in the target cluster. After this KIP, the current value of follower.replication.throttled.replicas would be kept in the target cluster unless the target broker's version is older than Kafka 2.3.0. 

means we would have the fallback logic if the broker is incompatible with IncrementalAlterConfigs API. This setting will be marked as deprecated immediately. 

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