There used to be a chat icon in the header indicating that you have unread chat messages. This was very helpful to notice activity in other chats while you were looking at a specific chat. For example, to notice that person B replied while I am chatting with user A.
That icon was removed: FIX: hides redundant chat icon on mobile chat routes by yuriyaran · Pull Request #35015 · discourse/discourse · GitHub
And while I understand that you don’t need a button which takes you to chat while you are at chat, I do really miss the notification count that was lost when the icon was removed from the header.