Harmonics issues was and still is much of an issue it you have pretty intense neutral loading by non linear loads on wye systems like maybe a data center. They were more likely having more issues with service and feeder neutrals more so than the typical 20 amp "full boat" branch circuit in most cases as well. Not that the neutral wasn't seeing higher current issues but the reality also is with 240.4(D) small conductor rule the load on the conductors was more limited than for conductors over 10 AWG - A 12 AWG is rated 35 amps @ 75C, but we limit it to 20 amp circuits for most situations because of 240.4(D).
IG's were typically a result of data errors when transmitting information between items back then, most the common data transmission methods used for nearly 30 years now doesn't even have a connection to a grounded conductor or cable shield to create those same type of errors should stray currents be present on the ground or shield so isolating EGC's has no effect on the data transmission method anymore.
