Good question. At least on the surface it seems only fair that at some point, after the archivist has performed a reasonably thorough search for copyright maintenance, the burden of justification then switches over to the (latest?) copyright claimant. "Yes, we have the internet now. Yes, if you intend to enforce your copyright, your claim should be searchable via the internet. Either way you may still have to reach out and publish notice of your claim." That kind of thing.
Kind of like Gutenberg.org texts, with their disclaimer that "we checked and couldn't find any renewal of copyright," or whatever it says nowadays...