Totally, but JWT-like blobs can be detected (see sibling comment), and parsing attempted, so for the automated scanning use case, if I understand correctly, it

Totally, but JWT-like blobs can be detected (see sibling comment), and parsing attempted, so for the automated scanning use case, if I understand correctly, it can be done with perfect accuracy, just at a larger computation expense and worse security exposure due to the complexity of the scanning and the need to parse. The more interesting side to me is the benefit to humans, from the prefix technique.