Requested feature overview description.
New metatags like added:foo and removed:foo that can make search result shows only when the tag is being added or removed.

Why would it be useful?
Easier to research a tag's usage. Like which posts have been tagged with that tag, or who removed that tag from every post.

What part(s) of the site page(s) are affected?
https://e621.net/post_tag_history/index

That would same some time. Though the current method of searching for the tag and then just text-searching the results for '-(tag)' or '+(tag)' is not too bad.

+1 for any way of searching on tags that have been deleted from posts (not necessarily through metatags).