In order for a work protected under copyright to become abandonware, the rights holder must actually grant permission.
And the person stating that the work is now freely distributable must actually have the legal right to act. And that gets complicated, if the intellectual property rights are held by an entity which no longer exists and all its assets have been transferred to another entity.
You can't take a single ambiguous statement or promise from the purported author at face value unless you can be sure that he or she possesses the legal right to make the entire work freely distributable.
Note that copyright on different components constituting software may be held by multiple parties.
So the author John Smith who wrote the main program but licensed the graphics or other code from another party may only have the power to maike some of the work freely distributable.
Or the author of the software might have created the product for his employer in which case he or she lacks any right to release the work for distribution.
A logo in a piece of old software might also be protected by trademark law, and though it's unlikely that anyone would sue, you can't really know who owns what.
Copyright law does not recognize waiver by silence, even if the copyright holder fails to send a takedown notice, this lack of action does not create any enforceable promise not to sue for violation of copyright.
The forum is skirting on thin ice if it assumes that there is a de minimus or grey area wherein some copyright infringement won't be actionable if it respect nice takedown requests.
Sorry, but that's how copyright works.
Copyright and other intellectual property law is necessarily a maximalist regime of legal repression.
Unfortunately I think you are dead wrong about Brexit, the UK government is in no mood to lessen the laws making even non-commercial sharing a criminal offense, or to abandon the censorship
of overseas websites.
On the contrary, Amber Rudd wants to backdoor encryption, and outside the European Union, you are no longer protected by the human rights judgments of the CJEU.
The CJEU gets lot of wrong about copyright, but it struck the data retention directive, which required the members of the European Union to log their citizens' internet trafick.
Outside the EU, you will still get encryption backdoors, bogus prosecution of non-commercial file sharers and extension of the Fraud Act to copyright infringement.