The tag license-key was first used on November 14, 2011. Despite being around this long, it's only got twelve questions, of which two have negative vote counts.
Perhaps one could say that license keys are a niche subgroup of signature-like things and whitebox cryptography, but it seems to be a honey trap if anything: By having it, we encourage people to ask low-research questions. A “license key” is not a concept in cryptography and while doing research on ways to get there, you inevitably stumble across the cryptographic methods to get to a possible solution.
There's room for saying that at least it acts as a containment tag of sorts, but it also seems to overlap with copy-protection to me, which itself only has seven questions. The copy-protection tag is a superset that includes the license-key tag, which frees both the answers from having to strictly stick to license keys, and encourages questions to explore alternative venues of (possibly more effective) copy protection methods.