I noted there is only one question with the polygon tag, and is it closed. This suggests the tag is not really that useful, and it might be a good idea to remove the tag.
Question:
When there are tags that are used only on very few questions that are all closed and not locked (e.g. the polygon tag), should such tags be removed?
Personally, however, I think the best option is to remove the tag and let it be cleaned up. This is personal opinion, not Word of Mod.
According to the help page, tags are for
sorting questions into specific, well-defined categories.
In addition, tag wikis are a useful attachment to tags to help clarify when they should be used.
However, in this specific case, questions relating to polygons can be found by literally searching for is:question "polygon", and the use case is incredibly obvious. As one of the more prolific tag wiki editors, a tag wiki for polygon would read something like
To be used on questions related to polygons, a closed section of a plane bounded by straight lines, and their properties.
similar to the polynomial tag and wiki.
While this is, inherently, not a negative against creating a new tag, adding polygon to 80+ questions would flood main with a bunch of years-old inactive posts to edit in a tag that isn't super useful.
Additionally, of those 80+ questions, over half of them already have the geometry tag, which is close to being a tag synonym of polygon already.
Therefore, I think the best option here is just to delete polygon from that question and call it a day there.
[polygon] with [geometry] on the question could also be a good idea \$\endgroup\$