Is discarding used sanitary pads (for menstruation) permissible or forbidden, since some consider that whoever sees the blood in them is like one who has seen the 'awrah (private parts)?

There is no objection to disposing of sanitary pads soiled with menstrual blood in trash cans, as this is not part of a woman's awrah (private parts that must be covered). Menstrual blood is considered an impurity (adha), as stated in the Almighty's saying: "And they ask you about menstruation. Say, 'It is an impurity [or a harm].'"