What year did the NEC (US) require all new construction switches and outlets to have ground screws?
Technically it happened about the same time that UL started listing non-metallic boxes for residential use. When the boxes ("handy" boxes) were steel, any device body was grounded by virtue of the mounting screws. Once the boxes became non-conductive, the devices that REQUIRED grounding needed to have grounding screws. When I started as an electrician in the mid 70s, the code stated that for metallic boxes, you had to ground to the box but for non-metallic boxes (at that time they were still "bakelite"), you had to ground devices that "required grounding". That is still the way it reads to this day (250.148).

"One or more equipment grounding
conductors brought into a nonmetallic outlet box shall
be arranged such that a connection can be made to any
fitting or device in that box requiring grounding."

The tricky part was, when did switches start "requiring" a grounding screw? That changed only when UL started requiring ground screws on switches, some time around 2010. When there was no ground screw, there was no "required" grounding.