I've used msgpack4nim for serialization between Nim & Python without any issues.
Sometimes a repository is just "done" \- the msgpack protocol is not evolving anytime soon - so there's not much activity to be done. Otherwise there's <https://github.com/PMunch/protobuf-nim> . If you want Nim <-> Nim serialization then maybe plain Json serialization such as <https://github.com/treeform/jsony> can do the trick
