Found out that telnet as been removed in Mac OS High Sierra 10.13 when I upgraded last night, which is bad for me as i work in networking and some legacy equipment still only has telnet. You can enable it with Homebrew. First, update or install Xcode from the App Store.
brew tap theeternalsw0rd/telnet
brew install telnet
If Homebrew is not installed, it's available here >> brew.sh
Another option is to copy it from a backup, or other machine, if you have one into
/usr/local/bin/
--cheers