If you’d rather archive old forum posts than delete them (instead of deleting them permanently) then yu can create a special archive board and move old topics there.

First in Admin -> Boards you create a new board called “Archive” and unclick the appropriate boxes to make it viewable only by Administrators. Then to move a topic you click on the topic to move and scroll to the bottom of the page, on the left you’ll see a “Move Topic” button, click that and select to have it moved to the new Archive board.

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