The MUD community, while perhaps smaller these days, still thrives in certain corners of the internet, and is especially vibrant in the forums of several websites devoted to mudding. The forums cover coding, building, game design, administration, player relations and many other topics that pop up in online gaming. Here are a few sites that host forums:
MUDBytes - This site serves as an all-around repository for all things related to mudding, including codebases, code snippets, areas, intermud communications and more. The forums tend to be heavy on the developer side of things, so they are good if you are trying your hand as a MUD administrator.
MudLab - Forums are this site's speciality. In fact, forums are the only thing you will find here, and all of them concern themselves with some aspect of MUD administration.
Top Mud Sites - Like the forums above, here you'll find discussions regarding world design, coding, legal issues and more, plus forums advertising for players and staff.
MUDListings.com - Because the site is relatively new, having been first announced in October 2017, MUDListing's forums lack the breadth and history of the earlier sites. Nonetheless it has at least one feature that may interest you—a forum aggregator that lists not only recent posts from its own forums but posts found at third-party forums, including Reddit.
Reddit MUD - Though less a forum and more a link/discussion aggregator, this browser-based bulletin board hosts numerous threads that discuss active games, building tips, coding questions, differences in MUD types and so on. If you're looking for a MUD, or just want to talk shop, MUDders regularly visit this site.
The Mud Connector - Once the most visited mudding site on the internet, The Mud Connector revamped itself in mid-summer 2018, removing its old forums in favor of a Discourse instance.
That has since gone away.
Which is unfortunate. Despite being a common source of flamewars and hurt feelings, the old forums encouraged numerous discussions of building methods, code snippets, abusive players, gaming histories, and so on. At this time I've been unable to locate a full archive of these forums.
A (somewhat charged) explanation for its disappearance can be found at Reddit.
Grim Wheel is a site dedicated to old school text gaming.
I haven't yet finished or opened most of the links below. An asterisk marks each of these.
Buildcraft* - Articles which discuss the craft of building on Multi-User Dungeons (MUDs).
MUD Forums - Links to offsite forums that concern themselves with all things related to Multi-User Dungeons.
MUD-Dev - Where you can find archives of the former mailing list known as "MUD-Dev", as well as a link to the newest mailing list.
Imaginary Realities - Archives of articles from the now-defunct online magazine Imaginary Realities.
MUD Listings* - Websites which list the many text games still in existence.
MUD Clients* - Clients which you can use to connect to an active MUD.
Codebases - The various public codebases in existence today.
Area Archives - Areas for public codebases such as Circle, ROM and Smaug.
Miscellaneous* - Links that do not fall easily into any category.