A mention in a thread or forum post stays unread until you reach it in that thread or post, not when you read the channel. Replies that arrive while you look at a thread or post are read, a reply deleted before you saw it no longer leaves the thread unread, and Mark as read on a community, a category, a channel or a conversation reads its threads too.
A post or thread with new replies opens at the first one, under a NEW divider. Each joined thread and followed post in the sidebar shows a red @ count when someone names you, and has a menu: Mark as read, Notifications, Copy link and Leave thread or Unfollow post. A forum’s list and grid cards hide spoilers, drop formatting marks and show people by name, and so does the Pins panel.
A community “About me” holds 300 characters, as Discord’s does. The / list inside a thread or post follows that thread’s own slow mode and lock. In a Home direct message the menu says Pin message and Report message, and the # list no longer calls the channels of members who came by invite “Not joined”.