Not sure why everyone thinks that telnet is "not suitable for today especially in the case of security", although telnet over TLS exists. There are also certain things that telnet can do which SSH cannot (e.g. transport data for a block terminal).

And everyone who uses "telnet as a program for connecting to a server on any port and talking the protocol manually" forgot that telnet is a real protocol and this type of abuse only works if the telnet client is sufficiently dumb / doesn't negotiate parameters. There is a reason why netcat exists.