I seem to remember that at least some TELNET clients would default to "dumb byte pipe" mode if run on some other port than 23. However, I can't find anything supporting this memory, so quite possibly I'm wrong...
<https://manpages.debian.org/bookworm/inetutils-telnet/inetut...>
From the manual of the old Linux NetKit telnet: “When connecting to ports other than the telnet port, telnet does not attempt telnet protocol negotiations.”