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?
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.
If then the tag is the only one used for a question, that means the OPs are not able to classify their own questions. I would expect that at least a tag is easily found as SO questions are generally about a programming language, and in some cases they are relative to an operating system, or architecture. In the case a question is independent from the programming language, then there is language-agnostic.
In short, I think unknown should be removed.