curl -vu test:test --ssl -l ftp://localhost:2121/
* About to connect() to localhost port 2121 (#0)
* Trying 127.0.0.1...
* Connected to localhost (127.0.0.1) port 2121 (#0)
< 220 Welcome to Rclone v1.49.5 FTP Server
> > AUTH SSL
< 550 Action not taken
> > AUTH TLS
< 550 Action not taken
We don't currently support TLS connections in serve ftp.
It would be reasonably easy to add though since I think the underlying library supports it, it would just be a question of filling the correct options.