It seems that the functionality of "deleting" message keys was never implemented properly, but should be fixed in current OpenACS (oacs-5-10 branch). I have added months ago some code that prevents "deleted" message keys to be continued to be loaded, Hector implemented yesterday code to properly show deleted message-keys, allowing admins to "undelete" these, and provide proper statistics (in /acs-lang/admin/message-list).
