For example, for 1Mbps-type IC couplers such as TLP109 and TLP759 to operate, how many volts are minimally required?

In principle, as long as the voltage at the input LED side is equal to or greater than the maximum value for LED input forward voltage VF, IC couplers can operate. With the above also taken into account, as long as a power supply voltage equal to or greater than a standard DC power supply voltage of 3.3V is assumed, expected operation can be performed.