Since 23/tcp is a well-known IANA-registered port for the Telnet service, it is an RFC violation to use it for a service that is not telnetd/remote logins via TELNET protocol.
Any port below 1024 signifies that it is a "privileged port". This is an archaic distinction that developed in high-trust R&E networks, but it did signify that the listener on the port had administrative/root access to spawn a service there, so it was kind of a signal that you could "trust" the remote server with your login credentials.
The privileged ports were also priority, because if the unprivileged ones were "first come, first served" for unprivileged users, the administrator would have the ability to enforce the uniqueness of "privileged ports", and disable or kill any process that shouldn't be using one. A MUD Wizard who finds their port in-use (bound) on start is on their own.
Typically there were no MUDs running with, or needing, root privileges. They were run under user accounts, or specific unprivileged role accounts. They had no need of a privileged port, and many were clandestine or unauthorized, and forced to use a higher port number. That's why the 4-digit ports became so popular.
Anyway, the custom has already developed of blocking port 23 to protect users from unwittingly opening a management or login interface. Most shrewd admins would choose a port that isn't routinely blocked and filtered... and port-scanned.
If your favorite MUD runs on port 23 today, such as nethack or something, then I am glad for this change, which will force the administrator to select a unique port that does not imply privilege, TELNET protocol, or shell login credentials. It is totally RFC-compliant to select an unassigned port above 1023, and MUD conventions have popularized several numbers that are still recognizable to players today.
