I also had that bit appear on my screen and managed to remove it. At the same time as it appeared the forum list disappeared and the small tab (next to the search tab) that brings up the forum list menu has disappeared.
Do you mean the old “hamburger” menu that was next to the search icon at the top right of the screen? Click the new hamburger menu at the top left of the screen to expand the sidebar. Then click “All categories.”
Does that help?
You can also customize the sidebar to always include the categories you visit the most.