What is the .ssh/config corresponding option for ssh -N

So in ssh.c for OpenSSH 7.6p1 we find

            case 'N':
                    no_shell_flag = 1;
                    options.request_tty = REQUEST_TTY_NO;

so -N does two things:

• the no_shell_flag only appears in ssh.c and is only enabled for the -W or -N options, otherwise it appears in some logic blocks related to ControlPersist and sanity checking involving background forks. I do not see a way an option could directly set it.
• according to readconf.c the request_tty corresponds to the RequestTTY option detailed in ssh_config(5).

This leaves (apart from monkey patching OpenSSH and recompiling, or asking for a ssh_config option to toggle no_shell_flag with...) something like:

Host devdb
 User someuser
 HostName the_hostname
 LocalForward 1234 127.0.0.1:1234
 RequestTTY no
 RemoteCommand cat

Which technically does start a shell but that shell should immediately replace itself with the cat program which should then block allowing the port forward to be used meanwhile. cat is portable, but will consume input (if there is any) or could fail (if standard input is closed). Another option would be to run something that just blocks.

@thrig has the right answer based on your desire to do this solely within .ssh/config

One might also consider using a function with defaults to make other tunnel commands quick (especially if the tunnel does not change but only the host).

---

sshn() {
  # set your desired host and port forwarding as default
  # and allow them to be passed in if you should need it

  host="${1:-devdb}"
  port="${2:-1234:127.0.0.1:1234}"

  # now all you have to do is `sshn` and it will connect

  echo "executing ssh -N $host -L $port"
  ssh -N "$host" -L "$port"
}

---

Here are three examples of it in use: with no args the defaults specified in the function are used:

$ sshn
executing -N devdb -L 1234:127.0.0.1:1234

with tunnel default, run on a different host:

$ sshn host2
executing ssh -N host2 -L 1234:127.0.0.1:1234

with both defaults, run a full one-off to a new host/tunnel:

$ sshn host3 12345:127.0.0.1:12345
executing ssh -N host3 -L 12345:127.0.0.1:12345