...

1. Drain in-flight TX_PENDING:
   - Halt producers calling sendShareAcknowledgementsToTransaction
   - Wait for transaction.timeout.ms (default 60s) for any abandoned txns to clear
2. kafka-features.sh downgrade --feature share.version --version 1
Broker rejects the downgrade RPC if any in-memory TX_PENDING exists, preventing data inconsistency.
Software downgrade (binary): must follow feature downgrade; standard rolling restart.

Test Plan

Rough implementation [The actual implementation will be phasewise with multiple smaller PRs]:
    https://github.com/apache/kafka/pull/22357

The verification strategy focuses on state machine integrity and fault tolerance under high-concurrency and failure scenarios.

...