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Clients with no support for message format v2 will not be supported. In the rare cases where such clients are still used, they will have to be upgraded. Fetch request v4 or higher is required for message format v2, so fetch v3 and older would no longer be supported by the broker or Java consumer. Similarly, Produce request v3 or higher is required for message format v2, so produce request v2 and older would no longer be supported by the broker or Java consumer.

Rejected Alternatives

1. Maintain support for message formats 0 and 1. Message format 2 is required for correctness (KIP-101) and key features like idempotence and transactions (KIP-98).
2. Keep read-only support for message formats 0 and 1 when it comes to on-disk data to avoid forced conversion to message format 2 in Apache Kafka 3.x and later. This has some appealing aspects, but it would mean keeping all the code for handling older message formats for a long time. The benefit doesn't seem worth the cost given that message format 2 has been widely supported by Kafka clients for several years.