[not2b]

The technique has been used for at least two decades in hardware verification, though the terminology is different. If you search the literature, you'll find terms like "constrained functional verification", "coverage directed test generation", "functional coverage directed test generation", and the like. The technique is the same, random testing, with mutation to try to hit more and more coverage points.

[pfdietz]

It goes back af least that far in software, with the original fuzzing work from U. Wisc and McKeeman's "Differential Testing for Software". Those are blackbox techniques; AFL's advance was using a general grey box approach.

[not2b]

The hardware approach isn't blackbox, it explicitly uses the reachable state space and constraint solving to reach more coverage points, to do this the exact circuit representation is needed.