History of shared web pages in Safari
This was a long time ago, but I found out how to delete the recently shared items and haven't answered yet. So here is how it goes:

Delete the plist file:

Old: ~/Library/Preferences/com.apple.sharekit.recents.plist

New: ~/Library/Preferences/com.apple.preferences.extensions.ShareMenu.plist

I forgot when exactly macOS changed the file location, and maybe both file are present after an upgrade.

You can use plutil -convert xml1 &lt;filename&gt; to convert the plist into a human-readable XML format if you want to inspect it.