> [...] no, I am not confused, because I was giving an example of why the Telnet-implementation, the program, the client, was so inadequate for playing on MUD servers.

Then this is at the heart of our disconnect, because the post of mine that you originally replied to --- as well as, unless I drastically misread, the original article under discussion --- was concerned with traffic on port 23, the Telnet protocol port, and not with any particular implementation communicating on that port. The concern of my original comment was that this might affect MUDs that operate on port 23. Perhaps you can understand my confusion when you reply stating categorically that most MUDs do not use "Telnet" (meaning the program), when that wasn't really what was at concern (and therefore implied that my question had no basis).

It is a true fact that many MUDs operate on port 23. Many do not, but you can skim a MUD aggregator like MudConnect [0] to see that it is quite common. Aardwolf, Discworld MUD, and the IRE games --- which consistently topped TopMudSites (when that aggregator was still running, anyway) all operate on 23, potentially in addition to an unreserved port.

> what surprises me is that it is mandatory, and your clients don’t seem to interoperate without it? That is a strange reversal!

All telopts are disabled by default, per Telnet RFC; the only things you must absolutely parse under the RFC are the standard complement of NVT commands (such as IAC GA "Go Ahead"), even if they are otherwise implemented as no-ops.

Any input stream with the high bit clear is treated as pure data -- with the incidental exception of bare `\r`, which must always be followed either by `\n` or by `\0`; but Postel's Law has turned that into more of a guideline. So as long as the standard NVT encoding is assumed (which is just 7-bit ASCII) and the NVT core escape sequences are avoided, a modern Telnet-based MUD client can interoperate with a plaintext MUD server without issue. (As you know, this is also why people get away with using `telnet` (the program) to access HTTP and SMTP services instead of using something like netcat.)

Some MUD clients will eagerly send IAC DO / IAC WILL subnegotiations, but general practice is to let the server offer first -- probably precisely to ensure compatibility with MUDs that don't implement Telnet subnegotiations.

> Now as for the Diku, LP, and other “combat” type games, I’ve no idea

Diku-family MUDs are certainly the ones I have the most experience with. I understand LP MUDs also generally have Telnet support; or at least, I recall seeing a patch for them that MUD owners often sought to apply to their games.

[0]: https://www.mudconnect.com/cgi-bin/search.cgi?mode=tmc_bigli...