An excellent reminder! In this case, the encoding type tuple is an artifact of an earlier iteration of the code and now no longer needed at all. Joe and I thought it would be good information to keep around, but you're right-- it does break that API. Rather than requiring code at the call site, maybe another helper function that gets the encoding type from the current capability set (or default) and lets the user's code decide rather than just returning it is the way to go now that the information's not currently used. 
(Note: This is indeed a change in both kv and core; also affects riak_test.).