KIP-1274 — Deprecate and remove support for Classic rebalance protocol in KafkaConsumer

Accepted Kafka 4.3 Consumer

Deprecates the Classic (pre-KIP-848) consumer rebalance protocol in KafkaConsumer with removal planned for a future major version, making the new incremental rebalance protocol (KIP-848) the only supported protocol. The Classic protocol's eager stop-the-world rebalances and thick client-side assignor logic are known operational pain points; the new protocol has been production-ready since Kafka 4.0 and the Classic implementation carries ongoing maintenance cost with diminishing benefit.

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