Some badges have a reason associated with them: a link to the post that resulted in the badge, a description of the scenario in which it was earned, etc.
Other badges do not: the context is in the badge description itself.
Near as I can tell, tag badges fall into this latter category... Yet the pages listing them attempt to expose one anyway, resulting in an irritating "No longer available" placeholder:
If there's something that would conceivably be useful or even mildly interesting, put it there - otherwise, drop the useless text.