Is it obligatory to wash disposable sanitary pads before discarding them? And is it true that not washing them attracts harm from jinn and devils who feed on the menstrual blood remaining on them?

There is no legal basis that obligates washing menstrual cloths before discarding them. Rather, the apparent action of the female Companions, and their leaving them unwashed, is evidence that it is not obligatory, and the Prophet, peace and blessings be upon him, affirmed that. What is circulated about the necessity of washing them is nothing but superstitions.