This is a problem with copyright in general. There's no longer any requirement to provide information about copyright status, so there's no way for the public to know whether something is copyrighted (no doubt the uncertainty is intentional as an additional way to undermine the public domain). This is just as true for expired copyrights, or material placed in the public domain intentionally.
The solution isn't to add notices to non-copyrighted things; it's to require notice for copyrighted things, including date.
Individually, of course, one should simply assume things are not copyrighted if you have no way to know, and do whatever you want. If the author can't be bothered to tell you, you have no moral responsibility to care.
The solution isn't to add notices to non-copyrighted things; it's to require notice for copyrighted things, including date.
Individually, of course, one should simply assume things are not copyrighted if you have no way to know, and do whatever you want. If the author can't be bothered to tell you, you have no moral responsibility to care.