Hi,
the tag “traffic_sign=destination” is currently in use more than 3000 times. I have created the wiki page Tag:traffic_sign=destination - OpenStreetMap Wiki to document the tag, based on Tag:information=guidepost - OpenStreetMap Wiki .
The specification of the whole area of tagging of signposts, destination signs, waymarkers and similar things is a bit fuzzy and offers many alternatives, and few aspects have ever gone through a formal review process. I hope that the documentation of the status quo is a useful starting point for improvement.