All flex cables have a minimum bend radius, under which a specified number of fold-unfold cycles is guaranteed. This radius depends on the brand, but generally it's around 5-10 mm. Here's a spec for AWM 20941 flex cable from JUDD WIRE, which is sold on Amazon as a Raspberry camera accessory:
As you can see, you can bend it at a right angle with a raduis down to 5mm, and slide back and forth 10000 times, or fold it 30 times with a 5mm radius, or 100 times with a 1 cm radius.
What happens if you bend it with a smaller radius is not specified. In practice, this means that the cable may break sooner, or even instantly. It may also work, at least for a while. This is the same as neglecting any other rating: powering the Pi with more than 5.25V, heating it to above 85 degrees, operating at 100% air humidity etc. Of course, the fact that you only risk a $1 cable makes it less of a problem, if your project does not require high reliability and you can always replace the cable when it fails. Otherwise, I would stick to the spec.
And don't be fooled by flex cables you see in a laptop which are bent at very sharp angles down to 1mm radius. Those are likely to require special conditions during assembly, e.g. pre-heating the cable to 80-100 degrees C before bending. You won't be able to manually assemble laptops at home with the failure rate of an assembly line.