Apple, in their ultimate wisdom, have removed telnet and ftp client on macOS High Sierra.
It is true. You should not use these commands anymore to do unencrypted management on switches.
For a network administrator these are extremely important to do debugging and troubleshooting.
Get your telnet back:
First, install Xcode from the App Store.
Then open a terminal and get inetutils from the GNU site:
curl http://ftp.gnu.org/gnu/inetutils/inetutils-1.9.4.tar.gz -o inetutils-1.9.4.tar.gz
Then compile it yourself:
tar xvzf inetutils-1.9.4.tar.gz cd inetutils-1.9.4 ./configure make sudo make install
the clients are now in /usr/local/bin.
Preferably you install only telnet and ftp. make install installs a whole bunch of commands, which do not work as expected afterwards.