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This is a backwards compatible change. The major impact is how it affects the future evolution of the consumer group protocol. This is documented above.

Rejected Alternatives

None yetWe have considered expiring offsets immediately when the group stops subscribing. The risk is for dynamic groups which have subscriptions changing. If the only member subscribing to a specific topic falls out of the group, then offsets would be lost. Additionally, we wanted to make the behavior consistent with empty group expiration. From an offset expiration perspective, we can treat an empty group as a case where of the subscription is empty, which makes all offsets subject to expiration.