The face of a woman is part of hijab; it should be veiled from a non-mahram. She should cover her face with chuddar or burqah while going out or facing any non-mahram, she can open one eye for seeing the way, as it is narrated from Sayyidna Ibn Abbas and Qatada (رضي الله عنهما). It is mentioned in the Quran:
يا أيها النبي قل لأزواجك وبناتك ونساء المؤمنين يدنين عليهن من جلابيبهن
O prophet, tell your wives and your daughters and the women of the believers that they should draw down their shawls over them. [33:59]
It means to cover the entire body including face; since ‘Jilbab’ in Arabic means a chuddar which covers the entire body of a person. They should let the corner of shawl on their face so that nobody can see them. Obviously, after these details, there is no room to say that covering face is not wajib. The Quran explicitly commands to veil the face.
Allah knows Best!
Darul Uloom Deoband