How to enable telnet server on macOS 10.13.1?
Use ssh. Really - use ssh. But if you insist:

brew install telnetd


I'll leave this up since the typical homebrew services start script isn't implemented on the stable homebrew version. Once that's done, you can:

brew services start telnetd


The brew services needs a file like this in /usr/local/Cellar/telnetd/54.50.1/homebrew.mxcl.telnetd.plist and possibly the config file in /usr/local/etc/telnetd.conf or wherever.

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;
&lt;dict&gt;
  &lt;key&gt;Label&lt;/key&gt;
  &lt;string&gt;homebrew.mxcl.telnetd&lt;/string&gt;
  &lt;key&gt;ProgramArguments&lt;/key&gt;
  &lt;array&gt;
    &lt;string&gt;/usr/local/sbin/telnetd&lt;/string&gt;
    &lt;string&gt;--config&lt;/string&gt;
    &lt;string&gt;/usr/local/etc/telnetd.conf&lt;/string&gt;
  &lt;/array&gt;
  &lt;key&gt;RunAtLoad&lt;/key&gt;
  &lt;true/&gt;
  &lt;key&gt;KeepAlive&lt;/key&gt;