It depends on what you are referring to when you say "discussion board". Wikis may have discussions associated with specific pages, blogs, or users. They may also have a place where users can just come to discuss things not related to a specific page, blog, or user. For the rest of this post, I will refer to discussions associated with specific pages, blogs, or users as "comments" and the other type as "threads". All wikis have comments and threads. The question is which specific feature does a particular wiki use for its comments/threads.
Out-of-the-box MediaWiki (the core software behind Fandom wikis) uses talk pages for comments (example). A talk page is a page intended to host discussion about another page. Other then that, it is no different from any other page on the wiki. Users can edit the page however they want; including the content added by other users. Out-of-the-box MediaWiki does not have threads. However, there is an extension called DPLforum which provides a way to organize lists of pages such that wikis have an extra namespace for threads (example). Talks pages exist on all wikis but sometimes Fandom auto-redirects users to an alternative feature. DPLforum is available to all wikis but is disabled by default.
On the legacy platform, comments for pages in the main, user, and user blog namespaces are handled by a post-based system. The type of content you can add is the same as any other page but each user creates their own post and only they (and admins/mods) can edit that content. Relates posts are displayed in chronological order on a page. The page on which they are displayed is not the same as the page on which the content is stored. The content for each post is stored on its own page which cannot be edited except though the interface provided on the display page. The threads version of this is Special:Forum; what we are using right now. These features are available to legacy wikis but may be turned off in favor of the out-of-the-box features. Special:Forum and DPLforum can be used side-by-side if desired. Some legacy wikis have Discussions which will be discussed in the next section. Discussions and Special:Forum can be used side-by-side if necessary but Fandom had been pushing and is now forcing wikis to disable Special:Forum and enable Discussions. The dev wiki actually had all 3 thread features enabled until around mid-September when Special:Forums was disabled.
UCP is the new platform Fandom is pushing out. It uses a version of MediaWiki that is much more recent then the version used by the legacy platform. Since it is still MediaWiki, the out-of-the-box features are still available should wikis desire to use them. However, there is also Feeds. Feeds is UCP's version of the post-based discussion features. Discussions is the Feeds-based version of Special:Forum. UCP wikis cannot use the legacy post-based features. There are a number of differences between the legacy post-based features and Feeds. Feeds is essentially completely separate platform. It does not recognize any of the text markup used by MediaWiki. Therefore, the content you can put in a post is not the same as the content you can put on a page. Additionally, all content is stored in some sort of hidden database. These are just some of the differences. DPLforum and talk pages for the user blog and main namespaces can be used side-by-side with their Feeds-based counter parts.
What do you think?