As per nhahtdh's comment in the linked meta post, tag merging can be done quickly, but undoing a tag merge can be cumbersome. To be on the safe side, unless there is evidence that keeping the tags separate causes harm, I side with not merging the history tags.
Furthermore, anime-history is to manga-history as anime-production is to manga-production, and I don't think many people will be eager to merge the latter two tags under production. I imagine there are people who are only interested in the history of anime (maybe they only watch anime and don't read many manga) and vice versa, so even though the two histories are closely tied to each other, I argue that it is useful to separate them. Also, as キルア said in the comment, there are people who are experts in manga-history, but not anime-history, and vice versa.
Having separate history tags for different media also scales better. If you worry that this might lead to the creation of many other tags that don't fit exactly in those categories, we can maybe tag them all under other-history.