Telnet was a conventional over-the-network communication software. To-date, it has no encryption and SSH surpassed it to the point telnet is now deprecated. This section guides you on harden Debian by not using telnet.
These are the identified threats related to Debian software.
Telnet is a deprecated software installed by default.
Telnet is transmitting data between 2 nodes without encryption and secrecy protection including transmitting password in plaintext.
Here are the list if actions to counter the issues.
SSH is made available for many remote usage. Hence, you should uninstall telnet at all costs.
That's all for hardening Debian by not using telnet.