What is the response to those who say that many Muslim women in the past did not observe hijab, and cite as evidence Al-Nuwairi's mention that Aisha bint Talha did not observe hijab or cover her face, and that Aisha bint Abi Bakr did not disapprove of that?

If the negated hijab is the covering of the face and hands, then this is a matter of scholarly disagreement, and the preponderant opinion is that covering them is obligatory. However, if what is meant is the covering of the rest of the body, then one who denies this should be countered by stating that literature books are not a source for rulings of Shari'a, and that the obligation of hijab in this comprehensive sense is established by the Qur’an and Sunnah. Furthermore, what is stated in the book Nihayat al-Arab does not negate hijab in its broad sense; rather, it is attributed to Aisha (may Allah be pleased with her), and she did not cover her face only. This does not negate hijab in general, so Muslims should adhere to the truth they are upon.