Currently it is not possible to 'close' a channel without leaving it, or remove it from the sidebar. This can be problematic when you are a part of many channels, especially private ones where you will need to be re-invited to it after leaving it. For example, I would still like to be part of a particular private channel in the event that something comes up, but I would like to have the option to close it/remove it from the sidebar to avoid clutter.

Right now there is no method to close a channel from the sidebar and still be a member of the channel (for private channels).