It really shouldn't be. Struct packing is a purely technical skill, not really computer science at all; there's no general insight you gain from understanding it, and it's easy to look up and teach yourself if you need to learn it later. (Not that most programmers will ever need to do it; these days even many embedded systems have enough memory and cpu performance that this kind of microoptimization is a poor use of programmer time)
Not universally. None of my university's Java based curriculum or my brother's university's Python based curriculum mentioned this. My community college's C++ based curriculum mentioned it in passing.
I seem to remember it being mentioned in Bruce Eckel's excellent "Thinking in C++".
That said, I'm kinda curious how much this affects Objective C objects.
