I’m pretty sure that the question being answered is “How to find the probability of having a disease if you tested positive for it.” I’m observing people interpreting this to mean “What is the accuracy of the test?” which is not the same thing.
Maybe add a bit to distinguish the two questions?
My understanding is that neural nets already determine the key features that are important to the decision. The importance of a given feature is represented by the weight on a particular neuron/input-feature.
So no we don’t need every feature. We just need all features relevant to the decision. So some amount of pre-processing can definitely help.