When leaving rooms, they no longer appear in the room list (left column). If the last person in the room leaves the room, the room is still in the archive for 7 days, but is then permanently deleted.
As administrator:in, all room members should first be “kicked” (removed from the room), then the person is the last to leave the room and thus initiates the later deletion by the server.
An export of the room history e.g. into a .txt or .html file is not yet possible: https://github.com/vector-im/element-web/issues/2630