Customize sidebar… is a checklist of the views — Today, Activity, Later, Threads and the others — with Move up and Move down. What you take off waits under one More row, and your arrangement follows your account to every device and to the collapsed sidebar. On a short window the last views fold under More by themselves, so your conversations stay on the first screen.
A channel that holds a guest now stays under Channels; External connections lists only the channels shared with another organization. The workspace menu lists your other accounts and their workspaces straight after the workspace you are in.