Also related: if you use ControlMaster, then the master session takes over any port forwardings defined in new shared sessions. So you can “ssh host” once to establish the master, then “ssh host -L… -R…” to make port forwardings in new shared connections; the shared sessions can be closed and the forwardings will stay active so long as the master doesn’t close.

Is that documented as intentional? Seems weird to me that the process shutdown wouldn't tear down the forwarding