__run_keyed_callback defines the callback method on the first run and checks if it was defined (with object.respond_to?). This makes @_keyed_callbacks cache never used:
In order to check that I've add the debug code that indicates cache usage and run test suite for AS and AR - no usages found so far.
/cc @josevalim: sorry for that bug with object/instance context. This PR also fixes one more bug from previous PR.
AS::Callbacks.__run_keyed_callback: remove unused cache