Rav Yosef Messas, Collected Writings, p. 211
The prohibition is not from the uncovered hair itself, but rather from the custom of the daughters of Israel who were accustomed to cover their heads, because they thought at their time that this was modesty for a woman, and a woman who uncovered her hair was considered a breaker of the boundary of modesty, and for this reason the Torah warned all daughters of Israel not to do the opposite of the custom of the daughters of Israel in this. If so, now that all daughters of Israel agree that there is no modesty in covering the head…the prohibition has been fundamentally uprooted and has become permissible.
Rabbi Michael Broyde, 'Hair Covering and Jewish Law: A Response,' p. 91
The consensus of the Ahronim [late halachic authorities] for the last few centuries has surely been that there is an objective Torah obligation upon married women to cover their hair.
Rabbanit Dr. Meirav (Tubul) Kahana, 'At the End of the Day – Submission,' from Olam Katan, May 2019
At the end of the day, after all the discussion and clarification of the matter of head-covering, its value and significance, we also need to say simply and with submission that thus the Oral Torah taught us, that the basis of head-covering is a Torah-level obligation. It is so difficult to exercise the muscle of submission regarding matters that are not understood and clear to us. To simply fulfill them because so commanded the Creator of the world. Especially in our generation, thinking and enlightened on the one hand, connecting and feeling on the other — what isn’t understood or what we don’t “feel” remains out of bounds. Indeed, we must look deeply, clarify, investigate and understand; there is great importance in connecting with mitzvot and to fulfilling them in joy. But the beginning and end of all mitzva fulfilment is the aspect of doing the will of one’s Creator…
Rav Yerucham Fischel Perlow, Commentary to Sefer Ha-Mitzvot of Rav Sa'adya Ga'on, Aseh 96
But in the Talmuds it is explained that a Torah-level prohibition is learned from this verse (Bemidbar 5:18). Perhaps it is possible to say that the prohibition is not learned [directly] from this verse itself. Rather since we learn from the verse that the way of the daughters of Israel is thus [not to go bareheaded], we infer that it is an immodest matter that can lead to illicit relations. If so, it naturally leads to a Torah prohibition under the rubric of “Do not place a stumbling block before the bind” (Va-yikra 19:14).