The effective law will depend on the jurisdiction where it is sought to publish such works.
In the US works first published in the 1940s through the 1970s are protected for 95 years after publication, and so would still be protected. The US requirement for copyright renewal for works published prior to 1964 will not apply to works first published outside of the US due to the Uruguay Round WTO negotiations and the resulting Uruguay Round Restoration Act (URRA) which restored such copyrights.
In the EU most countries now have life+70 years protection retroactive to the Soviet era, but many apply the "rule of the shorter term" so that if the law in the country of origin applies a shorter copyright term, that will be used. (The US does not apply that rule.) A work punished in, say 1950 is likely to be still under protection, but not if its author dies shortly after publication, or even before publication. But a would-be publisher takes the risk that the work is still under protection, and if a current owner of the copyright comes forward with evidence that the work is still protected, the publisher may be subject to damages.
In both the US and the EU, and most other places, it is the responsibility of a would-be publisher (or re-publisher) to obtain permission if a work is still protected, and if such permission is not obtained it is no defense to a copyright suit that the publisher did not know who to ask for permission from.
On the other hand, copyright is everywhere a matter of private suits, and if no copyright holder comes forward and takes action, no government will enforce the copyright. But a publisher takes the risk of a previously unknown holder coming forward.