I don't find mention of max_mutation_size_in_kb in CHANGES.txt, but I do see it mentioned for 3.0.0 in NEWS.txt. It doesn't have (DataStax) doc either.
It looks like it was added on 8/19/2015: "patch by Aleksey Yeschenko; reviewed by Benedict Elliott Smith for CASSANDRA-6230": https://github.com/apache/cassandra/commit/96d41f0e0e44d9b3114a5d80dedf12053d36a76b#diff-b66584c9ce7b64019b5db5a531deeda1 It probably should be (or have been) added to CHANGES.txt as well. I do see that comments on this was added to the yaml file on 10/2/2015 as part of: https://issues.apache.org/jira/browse/CASSANDRA-10256 -- Jack Krupansky