...

Thus this change is proposed for a minor or feature release (specifically, 4.1 as suggested), and it will not affect any user.

One thing need to be mentioned is that DefaultKafkaPrincipalBuilder already implements KafkaPrincipalBuilder and KafkaPrincipalSerde. After this KIP, it will only implement KafkaPrincipalBuilder.

Now

public class DefaultKafkaPrincipalBuilder implements KafkaPrincipalBuilder, KafkaPrincipalSerde {
 // Implementation...
}

To-be

public class DefaultKafkaPrincipalBuilder implements KafkaPrincipalBuilder {
 // Implementation...
}

Test Plan

Given that existing KafkaPrincipalBuilder implementations used with KRaft already implement KafkaPrincipalSerde, this change should not result in new compile-time errors, and existing test cases are expected to pass.

...