...

If users delete a topic configuration in the source cluster, currently it would also be deleted from the target cluster due to the replace all behaviour of AlterConfigs API. However with IncrementalAlterConfigs, the topic configuration in the target cluster would potentially be left unchanged. Therefore any default topic configuration on the source cluster would need to be deleted from the target cluster.

The KIP also proposes adding the ability for users to choose between target cluster's default or source cluster's default when using IncrementalAlterConfigs for syncing topic configurations. For instance, users would be able to configure ConfigPropertyFilter to check if the configuration on the source topic is the default and choose whether or not to replicate it to the target topic. This gives users a bit more control in how they want to replicate their topic configurations. 

Proposed Changes

1) Extend ConfigPropertyFilter  class to have a new method called shouldReplicateSourceDefault which would always return false by default. User would be able to configure this method to return true in order to replicate source cluster's configurations with default values. 

...