but that unsuitable GCSE choices will hold them back regardless of A-levels.
I think that's a very strong claim. I've seen it made (I have a child who submitted a UCAS form including Oxford last month) but I've not seen much in the way of evidence for it being true. I thought the main concern was that some GCSE choices limit A Level choices, or make it less likely that you will do well in particular A Levels. It is absolutely the case that university admission for any over-subscribed university, and many rather less over-subscribed ones, is massively eased if you have the standard, expected A Levels for the course.
Given Oxbridge's enthusiasm for transparency over desirable qualifications, if it's true that GCSE subject choices are considered even if you are doing the "right" A Levels, it's surprising it hasn't been published as guidance.
It would be interesting to know how many people take what might be summarised, by some on this thread, as "weak" GCSE choices, go on to study traditional A Levels in which they obtain decent ASes, and then think they have been penalised for this when applying to university.
I can imagine the Oxford process, which doesn't look too much at AS but uses GCSEs more heavily, might have a whiff of this, but if they are weighting by subject that should be admitted to. The Cambridge process is heavily based around AS, including looking at AS module UMS scores, and they are apparently less concerned with GCSEs.
I doubt m/any Redbricks are concerned with GCSE subject choices beyond the core subjects.