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.

But …

For a network administrator these commands 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 the ⇒GNU inetutils using curl :

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

The next thing would  be

sudo make install

then all clients are  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.

To get only telnet and ftp, simply copy the executables.

cd inetutils-1.9.4/telnet
sudo cp telnet /usr/local/bin

cd inetutils-1.9.4/ftp
sudo cp ftp /usr/local/bin

If you require telnet for troubleshooting, ⇒netcat is a perfect alternative.