I have two notifications that have been sitting in my "tray" forever. The first is an "<app> has been installed" notification. The other is "Additional fonts required". Both were sent by Software, and both have outlived their usefulness.

Wherever possible, notifications ought to be revoked when they are no longer useful, in order to prevent the user from having to manually remove them themselves.

In the "<app> has been installed" case, the notification could be removed when:

 * The installed app is launched from Software
 * Software is closed by the user

Maybe there are other conditions also.

"Additional fonts required" is possibly more tricky. Ideally it would be removed when the user closes the app that requested the fonts, but I'm guessing that's not so easy.