In my answer to the question 'What should we define an uncommon language as?' I proposed the creation of the tag historical-language-variants. This tag could cover both Latin (which is normally considered "dead") and older language variants such as Old English and Middle English. However, this tag would be 28 characters long, while Stack Exchange limits the length of tags to 24 characters.
So what would be a good alternative? The only one I can think of now is dead-languages, which would imply that Middle English and Old English are considered "dead".