It doesn't seem the /post/tag_history.xml API function is working. When I request it with any combination of parameters (or no parameters), I just get a blank 404 response.
Two examples of requests that should return results, according to the API specification:
http://danbooru.donmai.us/post/tag_history.xml?post_id=366064
http://danbooru.donmai.us/post/tag_history.xml?limit=10
Updated by LaC