Is it considered unlawful to pluck the eyebrows, especially given the observation that some women who wear the hijab do so?

The ruling on shaving or thinning eyebrows has previously been stated as prohibited. It is not permissible for a Muslim woman to do so. The sight of veiled women doing this is not to be relied upon, because the rulings of Islamic law are derived from the Qur’an and the Sunnah, not from the actions of individuals. Sharia is the scale for the actions of people, not the other way around. A Muslim woman must derive rulings from the Book of Allah and the Sunnah of His Messenger, and take the Mothers of the Believers as her role models.