Having to maintain both sync and async modes complicates developments on apisonator. I'd like to get rid of the sync mode since the 2.15 already includes the async mode as default and there are no complaints AFAIK.

We have kept the sync mode until today just in case some error happens and we have to rollback. But if that happens in some future release, we can tell the client to rollback to use 2.15 and use the sync mode, since apisonator doesn't include any new feature that would make it incompatible with 2.16.