In all honesty, I'm probably a few years out of date. I used to advise applicants in a voluntary capacity, and that was what we were told during a UCAS training session. I've just trawled through the openly published UCAS terms and conditions, and can't find much. I did believe it was an option for the unis, not that your application would be automatically withdrawn.
I did find this page from UCAS that seems to vaguely suggest that unis can change your offers if you change any details of your exams (in the changes to exams and courses section). If you email the address provided, UCAS can probably/possibly give more specific advice. If the issue affects you directly, then having something in writing from UCAS would be very useful anyway.
I've never seen a real life example in relation to an unconditional offer (possibly because this has become the received wisdom, and there weren't so many given out when I was advising students) but I have seen a case where an applicant technically met a conditional offer, but it was withdrawn because they'd dropped an additional subject they were also taking. UCAS said this was allowed. I think it depends on how full the course is on results day.
UCAS do say that unconditional offers can be withdrawn because an applicant, for example, fails a DBS check. To my mind, this is a conditional offer where the condition is passing the check, but it shows these offers can be withdrawn in some cases.
I think if an applicant totally dropped out of Y13 following an unconditional offer, some unis (e.g. Birmingham) probably would try to find a way not to take them. However, the sort of student who tends to get these offers is probably not the sort of student who would do that. They probably also have supportive parents who wouldn't let them.