I’ve ridden up Rosedale Chimney Bank in North Yorkshire a few times, once with saddlebags full as I was youth hostelling at the time. This hill gets to 33% in places, and where it does, its fairly straight, you simply can’t zigzag uphill to ease the gradient becuase its so steep that it just turns your front wheel downhill when you switch zig to zag, or vice versa.
If this road is damp and you have high pressures, say 140 lbs, then your wheel will spin, seen it, done it.
The main problem is that your front wheel lifts if you are not incredibly smooth pedalling, and the angle of the hill is so acute that its difficult to get over the top of the bottom bracket because your handlebars are so much higher than the rest of the bike and effectively rotated backwards.
I have also ridden Hardknott Pass, which is also very nearly as steep as Rosedale, but longer, and to keep the front wheel down I had an handlebar back which I filled with rocks, this worked fine but made the steering somewhat interesting.
Given a low enough gear, and a highly modified frame along with low pressure tyres, you might be able to get perhaps near to 40%,but on a racing cycle I think that Rosedale and 33% is pretty near to the limit.
I should perhaps add that Rosedale is rather steeper on the inside of the notorious hairpin, steep enough to lift the wheels on cars which have limited suspension movement, but realistically this part is very short, and if you time it right you can take the longer way around it but you then are going on the wrong side of the road so you have to watch out for traffic.
This picture does not really do it justice.
This is better, but this is not the steepest part, and the bad hairpin is just at the vanishing point of the road as it sweeps around left.(you can see the rider cheating by zig zagging across the road, around that top corner you can’t do that any more and you tackle it dead on)
The grassy bank that runs upward from centre upwards to the left is a very good indication of what lies ahead as that is the road edge.