Can I use ssh in "forwarding mode" as a workaround missing Telnet in High Sierra?
Well, using built in utilities the answer would be nc. nc is easy to use for this, a simple nc &lt;hostname&gt; 23 does the job. Obviously, if your telnet host is using a non-standard port then replace 23 with the port number. &lt;hostname&gt; can be either the name or IP address.

nc is the BSD version of the Gnu/Linux netcat utility. If you want the Gnu version it is available via homebrew, brew install netcat. If you're going to that much trouble you may as well install telnet from Sierra on your High Sierra Mac via homebrew, brew install telnet. Note that you should never run brew via sudo.