Both Project and Independent Boards are archived in the All Boards section. This section can be accessed in 2 ways.
The first way to access the All Boards section is to click on “Boards” in the left sidebar.
The second way is to click on “All Boards” under the search bar in the section that opens once you hoover over the “Boards” field in the left sidebar.
Once in the All Boards section, a Board is archived by clicking on the “Archive” button in the Archive collum.