There must be a minimum standard, maybe the equivalent of 3 D's 🙄; surely you shouldn't be able to take a degree with less than that!
Back in the days of the Ark when I took O Levels a friend of my brother's was given an offer of EE, by Oxford for maths.
Virtually an unconditional, but you needed 2 A Levels to get a grant.
It was very rare but it did happen. Also universities might make offers to different students for the same course.
If we did switch to a system where students applied after results then all offers would be unconditional.
There are already lots of students applying with known results eg those going via the BTEC route, or access course or students who have taken a gap year.
It's not the unconditional offer per se that are the problem it is the number of them and the pressure on students to make that offer their 'firm'.
IMHO unconditional offers should only be made after an interview, entrance exam or portfolio and should be decided by the academic department not the admissions office.