The fact that untagged is shown among new tags made me wonder how it got there.

This is the only question with this tag:

Here is revision history and timeline.

I see in Google Cache that is was tagged concavity.

Has the tag concavity been removed recently? Isn't it more usual on MO that tags are replaced by tag-removed rather than untagged (as mentioned here and here)?

I tried this SEDE query. It found only this question as a question having the tag concavity.

So it seems quite plausible that the tag was removed by the tag-pruning process, which removes single occurrence tags. (I have recently asked on another meta: Is it possible to find out whether a tag has been removed by an automated process?)