JSON-RPC is not removed, but actually current WebUI is also using it so there were some changes in permissions.
Fully functional JSON-RPC will be returned with RutOS 7.2 (activation deactivation via WebUI - Administration > Access Control)
Now there is a workaround, by changing rpcd config via SSH, but we do not usually give it out, since WebUI also rely on it, so it is easy to brick WebUI access to router. If you need it, contact me via a private message.
For users, that just want to test JSON-RPC out I would offer to do that on older OS versions (RUT9_R_00.06.08.5), and wait for 7.2 release.