Now that Scala 2.12 has been released, we should drop support for Scala 2.10 in the next major Kafka version so that we keep the number of supported versions at 2. Since we have to compile and run the tests on each supported version, there is a non-trivial cost from a development and testing perspective.
The clients library is in Java and we recommend people use the Java clients instead of the Scala ones, so dropping support for Scala 2.10 should have a smaller impact than it would have had in the past. Scala 2.10 was released in January 2013 and support ended in March 2015.
Once we drop support for Scala 2.10, we can take advantage of APIs and compiler improvements introduced in Scala 2.11 (introduced in April 2014): http://scala-lang.org/news/2.11.0
Link to KIP: