Sometimes, people remove the better_version_at_source tag from posts that legitimately have a better version at the source.

How can I find these?

I want to search for posts where someone has added the bvats tag* and then somebody else removed it later.

And how can I find posts where the bvats tag was already included at upload time?

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*i.e. it wasn't already included in the tags at upload time, which happens when somebody uploads a better version and just copies the tags from the inferior version

The post tag history is pretty much not capable of this right now. The fancy history everyone is used to seeing is generated from multiple rows for display purposes, but the data isn't laid out in that format. There is only a list of tags for each revision. Searching for "X" will only find posts where X exists on the post for that revision. Short of rewriting the tag history entirely, this seems implausible to try and implement.

This is considered as tagging abuse, so I do encourage anyone seeing this happening immidiately reporting the user - also any other kind of similar removal of valid tags.

I would highly encourage those removing the tag thinking it's not needed anymore to also include reasoning as there is field for this. One of the most absurd things that I have bumped into is that twitter actually does store PNG files for every image they have, but if served image isn't PNG, then the PNG version is simply JPG version resaved as PNG. Many of my tags were simply removed as users and staff saw the version matched the source and both were PNG...

So you have for each post a series of snapshots of what all tags they had in that instant, and each has a timestamp? So basically go back through them, and you're looking for where you don't have the searched tag, and then you do, and then you don't again, and you want at least one instance of that to include it in the result set? That seems simple enough, but it seems like it doesn't totally answers the question you're trying to ask. Do you only want ones where they don't currently have that tag? Like it got added and removed at some point but exclude ones that also have it now? And do you want every instance of removal separately, or just the most recent? Defining how that ought to work and what it actually tells you when you find it would be the harder part. It could come and go several times over a post's history and have been correct each time because of changing availability or something like that.