I'd say 2.4 volts (the voltage produced by 2 NiMH batteries) is the reasonable lower margin for today's standard microcontrollers, and digital circuits.
sure there is some new low voltage FPGA but it is not easily available, and you can not just solder a small IC, connect 2 batteries, and it will do something. so it counts differently.
also LEDs need at least 1.5 volts to 2.5 volts.
I've encountered single cell AA supplies inside minidisc player, and scent dispenser.
they contain dc/dc converters.
2.4 volts devices can work directly from the NiMH batteries, as they will retain that voltage over most of the discharge curve.
so it is a very economic threshold.