"un-" is indeed a funny prefix. You might enjoy (or be frustrated by) this analysis… https://hardmath123.github.io/un-unravelable.html
In reality of course the base state is reversed: having no license means you are allowed approximately nothing, and a license gives you (often limited) rights to do something with it. In reality an un-license would take away rights you previously have been granted.
It feels like we are on the threshold to license becoming a Janus word.
Excerpt from https://www.gnu.org/licenses/gpl-3.0.en.html