digital-signature is obvious and clearly defined (though, it still needs a description).
It's signature that is ambiguous. Almost all questions under that tag use it as a synonym for digital-signature. There are only a handful of questions that do not have to do with digital signatures (only some examples included):
Questions about signatures at the end of emails
There is already a tag for this type of use: email-signature
Questions about handwritten signatures in PDF documents
Such questions are rare, and I don't see anyone on SuperUser being an expert in "physical signatures", so I think the tag is unnecessary.