Copyrights, unlike trademarks, do not expire if you do not defend them. Thanks to Disney, for everything published after Mickey Mouse, copyright would never ever expire. If it's made after 1926 and not explicitly released into the public domain, even if the original creator no longer exists (dead in the case of a person or dissolved in the case of a firm), chances are someone, somewhere, owns the copyright.
Now, I'm not a legal expert, but I heard it from an U.S. lawyer that only the entity whose copyright is perceived to be infringed (or its authorized representative such as the RIAA etc) can actually take the matter to court or write a meaningful nastygram to the hoster or whatever.
(In Russia, there's an agency run by a senile fat bastard that owns the copyright on your thoughts and your kid's DNA, and everyone breathing or having drawn a breath at some point in the past from the creation of the world 7600-something years ago until now retroactively owes the fat bastard copyright infringement fees because they might have broken, or might break, copyright. I manage an underground record label and maaaaaan do I hate the fat bastard. You might have heard of his attempt to fine Deep Purple for performing their own songs without his express permission.)
If the copyright holder is unknown or does not care, the illegal publication can theoretically sit where it is forever.
However, and this is important, everyone can harass and troll the illegal publisher over the infringement. Still more importantly, if the publisher does not make it their business to check and double-check and triple-check the abandonware status of each and every link, they are bound to run into trouble sooner or later - even if they do their job perfectly, because an unclear copyright status is inherently risky. So a rule of "no links to illegal publications, ever" is intended to be a wallet-friendly alternative to having a legal team for checking copyrights and dealing with trolls.
Of course, not you nor anyone on GOG would be fined or serve time over that link. But if they were to pull the plug on shady links, the law of large numbers would dictate that actual Guillaume-hours would be spent on telling trolls to shut up already instead of signing up new publishers and actual dollars would be spent on legal fees in place of coffee and bagels. And no one wants that. Therefore, no links.
