For some rather inexplicable reason, Apple decided to remove the
telnet utility on OS X High Sierra. At first I thought I must have just been going crazy when it disappeared, but a quick Google showed me that yes, this really happened.
I built it from source, and you can download my build. Just unzip it and put it somewhere in your
$PATH and it should be usable as per normal. Here are the hash sums of the binary (meaning, the thing you get when you unzip the zip file) so you can be safe: