For libraries that rely on data.json the 0.2 release was a major unexpected breakage. For example, for
Monger to support 3 JSON serializers (Cheshire, c.d.j 0.1.x, c.d.j 0.2.x) we had to do this:
https://github.com/clojurewerkz/support/blob/master/src/clojure/clojurewerkz/support/json.clj

This is some of the craziest code I've seen. While writing it, I realized that several changes
in the c.d.j. API were easy to shim with a backwards-compatibility function. clojure.data.json/json-str
can be implemented on top of the new function easily, for example.

clojure.data.json demonstrates very questionable library maintenance practices and the 0.2 release
already cased a lot of confusion and pain to the community. There was 0 communication about the changes upfront, this shows lack of respect to the community and care about backwards compatibility.

So, bringing back a shim API layer for 0.1.x compatibility is a must. There are many codebases on clojure.data.json 0.1.x and their developers probably are not going to stop doing what they are doing
and deal with unnecessary c.d.j. API changes. Other library maintainers that extend or depend on c.d.j.
are caught between a rock and a hard place.