It has? I've been under the impression that the "solutions" are "trust us, we don't allow that" (relying on an authority with full knowledge, as partial knowledge isn't sufficient) and "use more resources than anyone can feasibly contest" (bitcoin).
You could build a merkle tree to say "we exist after X" but not "there is no other X". And publishing that tree for verification would seemingly violate "zero knowledge", unless you know of some way to scrub that, and also hide timing information, because timing information can identify visitors to observers.
You could build a merkle tree to say "we exist after X" but not "there is no other X". And publishing that tree for verification would seemingly violate "zero knowledge", unless you know of some way to scrub that, and also hide timing information, because timing information can identify visitors to observers.