It seems that there are 46 questions tagged unknown on SO. Looking at them, I'm not sure the tag really adds anything, except maybe for the "What programming language is this" questions. Do you think any tag removal is in order?
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Giving a quick look at the questions tagged unknown, I noticed that some of the questions already contain that word in the title, one question uses that tag and it is asking to identify a programming language, and one uses that tag for lack of a better tag.
I could probably understand using unknown together language, but it is definitively not a good idea to use such a tag, especially when so different questions use it.
In short, I think unknown should be removed.