I believe this was the last thread on that topic: Notes on Stan Output Serialization Options (YAML, Protobuf, Avro, CBOR)
There was my design docs on JSON output that we closed because of the issues discovered of handling Nan/Inf in the most popular R and Python JSON libs. We would essentially had to maintain different dialects and maintainer of the R jsonlite package denied the feature request to unify with Python’s libs.
Other than that AFAIK no work is going on here, at least no one has mentioned it.