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Users who have not and cannot upgrade to Scala 2.10 11/2.12 and are still using the old clients would have to continue to using use Kafka 0.10.x until they can upgrade to one of the new more recent Scala version or move update their code to use the recommended Java clients (which is recommended anyway).

Rejected Alternatives

1. Support 3 Scala versions in the next major release, 2.10, 2.11 and 2.12: the cost seems to outweigh the benefits.