Telnet has been removed from Mac OS High Sierra, but can still be useful for developers to check ports are open etc.
If you need to enable it, you can do so with Homebrew:
brew tap theeternalsw0rd/telnet
brew install telnet
Update (10/10/17): I received an email from David Waitzman in which he notes you can get telnet from an old Time Machine backup and then copy it into /usr/local/bin – I’d recommend that options too if you have a backup.