Dear Maintainer,

Illustration : try to right click on xfdesktop's background : you don't have "open terminal"
Try to do it in thunar : you have it.

Try this on stretch's xfdesktop : you have it.

I asked on xfce forum and they replied :

I want to appeal for an update on this, because it's kind of cool to have for people trying to "pimp" their actions for beginers purpose.
But it might be too late :/

Cheers.

Well, on my case, thunar's custom actions DO appear on
xfdesktop4 menus (when their required apperance conditions are met).

Sophoklis