brew install telnet
Warning: No remote 'origin' in /usr/local/Homebrew/Library/Taps/homebrew/homebrew-services, skipping update!
Running `brew update --auto-update`...
==> Auto-updated Homebrew!
Updated 3 taps (homebrew/cask-versions, homebrew/core and homebrew/cask).

You have 11 outdated formulae installed.
You can upgrade them with brew upgrade
or list them with brew outdated.

Warning: You are using macOS 10.14.
We (and Apple) do not provide support for this old version.
You will encounter build failures with some formulae.
Please create pull requests instead of asking for help on Homebrew's GitHub,
Twitter or any other official channels. You are responsible for resolving
any issues you experience while you are running this
old version.

telnet: A full installation of Xcode.app is required to compile
this software. Installing just the Command Line Tools is not sufficient.

Xcode can be installed from the App Store.
Error: telnet: An unsatisfied requirement failed this build.

tar -zxf inetutils-2.3.tar.xz
cd inetutils-2.3
make -j8 ## 報錯可不管
sudo make install

## 最后驗證安裝結果
telnet --version