The white conductor has been permitted as an ungrounded conductor in a cable assembly for at least 100 years. A 2-wire switch loop being the most common use of the white conductor as the hot leg down to the switch. This was easily identifiable in the lighting outlet box because the white conductor going down to the switch was connected to the hot leg at the box.
The 1999 NEC had a considerable re-write of Article 200 (Use and Identification of Grounded Conductors) which required the white conductor in a cable assembly to be re-identified when used as an ungrounded conductor.