"We can do nothing".
Sorry, I don't give up that easily, not even with multi-nationals like Apple.
Apple have crossed the line here, and I won't accept it, you shouldn't either.
There's a principle/fundamental issue at stake here.
High Sierra is out of support, Apple may no longer offer updates for High Sierra, but (and this is the crux) the page that tells you and millions of Apple users that informaton should show this clearly for the life of the machine. This version of macOS is currently operating against a string of UK laws including the UK's Equality Act.
The whole thing makes a mockery of the UK's National Cyber Security Centre for a start, why I brought it to their attention.
What's the point of having a very expensive high profile Government funded body to mitigate the effects of viruses and alike, when on the other side of the coin, we allow companies like Apple to causually trash the formatting of the App Store Update page, which means many non-techical types will just either not bother applying the updates that are there through this garbled mess or worse, be subject to one of those telephone tech scams.
For any Cyber security policy to work at the National level, uou have to have these two policies connected, and this shows they aren't.
As said, I won't let this lie.
Even if it means writing to both NCSC and the Competition and Markets Authority, because this also can be seen as an 'anti-competitive action' against third party resellers of second-hand Apple Equipment, making their job harder, by having macOS show garbled update pages, once a version of macOS is out of support, giving rise to those customers purchasing said equipment, returning the items in belief they are infected with a virus.
As others have pointed out, this is the perfect exploit to take advantages of non-technical folk, where people will be cold-called and they explain that the update page can be repaired remotely for a fee. An actually even technical folk, because I spent more than a week on this trying as many different scenarios, to attempt to resolve it.
Apple need to put this right NOW, this is no joke, it's a serious fundamental issue. Apple have crossed the line here.
Time is running out for Apple to correct this because as time carries on, it will be seen as deliberate by the regulatory authorities.