Back to the original question about the resistor symbol.
I think the zig zag style resistor is still part of the IEC standard.
And the box style resistor is in the IEEE/ANSI standards.
At least everything i found so far seems to tell me this.
The newest document i found: http://www.moeller.net/binary/schabu/wiring_man_en.pdf page 551.
An older document from 1975: https://www.noao.edu/ets/Mechanical/Policies/ANSI%20Y32.2-1975.pdf page 28. This document proofs that the box resistor does fulfill us norms. It also shows that at least back then the zig zag resistor was recommended by iec.
If my research is correct, it seems both resistor symbols are part of iec recommendation and should therefore be in the kicad library. It would be great if someone with access to newer norms could verify that.