Correct. JSONRPC support was introduced in EXOS 21.1.
For earlier EXOS releases, I have a kind of odd way to do JSONRPC over telnet session that I never published.
I have python code examples on how a server would use this.
Server:
 
- connects to a switch via telnet
 
- Authenticates (username/password) 
- starts a script tJSONRPC.py on the switch which takes over the connection. 
- communicates JSONRPC with the switch point to point over the dedicated session.
Switch:
 
- authenticates telnet client connection 
- accepts command to start tJSONRPC.py (run script tJSONRPC.py) 
- session communicates JSONRPC (method=cli) until closed.
Would you be interested in trying this out?