How to clear a recent documents list from Word 365 (v16.43) on macOS Catalina 10.15.7

From the File menu on the Menu Bar at the top of Word's window, choose Open Recent. There you will see a More option. Click it and the full Recent list appears.

Hovering your cursor over any item on the list and three icons appear. Click the Ellipse icon and there is an option on the contextual menu to remove the document from the list. BTW - don't choose Delete unless you want to erase the document from where it is stored on your system.

You have to do this one at a time.

There are several Library folders on your disk. The file you seek is in your hidden user Library folder, not the Library folder at the root of your disk.

While holding down the Option key, click on the Go menu (in the macOS menu) and choose Library. Then look in ~/Library/Containers/com.microsoft.Word/Data/Library/Preferences/ and drag com.microsoft.Word.securebookmarks.plist to your desktop. That deletes Recents that are stored on your disk. You'll still see Recents that are stored on OneDrive. To eliminate those, sign out of your account. (You only need to sign in once a month to keep your account active).