How do we actually know
whether any topics are in/out of the exam?
An example are nets - the popular consensus (appears) to me that they are not likely to be in it.
I found this out after teaching myself the subject & dc too. I am relieved as they are a headache, especially arrow directions, my head spins. The only consolation is that if they do unexpectedly turn up, dc has a fighting chance.
I think that this view, as with most things, comes from past experience & hearsay e.g. Past parents & tutors.
All we can do is gauge the probabilities individually, & balance our limited resources e.g. Time & money etc.
As a general guide, I like the old saying "if you fail to prepare, then one must prepare to fail" - so those doing mocks may be on to something