Not sure what is the deal with CRAN and versions, but just came here to comment about RStan3
PyStan (PyStan3) now follows the new API. Also, at v2-> v3 step, we upgraded the package structure (PyStan=frontend, HTTPStan=backend).
PyStan interface is very light and handles only Python side of things.
HTTPStan only handles Python-Stan C++ interface and is kept simple.
It might be that in future HTTPStan is replaced with some implementation of ServerStan.