PBA:
Professor Jonathan Sarna authored that historical study very recently. It is based on what Orthodox institutional practices were at the time.
Granted, times have changed, but we also cannot be revisionists. And, prior to R’ Moshe Feinstein and Rav Soloveitchik, who were very homiletic and polemical in their respective arguments on the issue of mehitza due to the height of Orthodox//Conservative competition in the 1950s (at that time, there was still little which divided the 2, since many American Orthodox Jews were exceptionally lenient then, and many ignored certain halakhot, and the Conservative movement then was rather frum), there was no mention of mechitza being a requirement, let alone something d’oraita, in any literature of the rishonim or acharonim.
In fact, the Igros Moshe’s argument derives the fact that mechitza is a chiyuv from Divrei haYamim, which violates a klal psak- we don’t derive halakha from sifrei Nach. Karaites do that, whereas Chagigah 10b tells us that this is not how we deduce halakha.
Furthermore, he says that the “tikkun” made on the day of simchas beis hashoevah (Sukkah 52) is the source for mechitza, as a mechitza was somehow made based on an obscure pasuk in Divrei HaYamim.
One does not derive Torah law from a Biblical book that is post-Torah, and the Chronicles were among the last books of Hebrew Scripture to have been written, by all accounts. Second, the idiom tiqqun, enactment, is a human made law that by definition cannot be a Scriptural obligation.
R’ Soloveitchik also claimed mechitza was d’oraita, citing Devarim 23:15. However, there are two problems with this approach. Rabbis living after the Talmud, however great they may be in wisdom and learning, do not possess the authority to derive new Biblical laws out of Biblical texts. Such claims require not only their formulation, but the review and endorsement of the Supreme Court of Israel, the Sanhedrin. This juridic power has sadly lapsed in our day. Since the Talmud does not explicitly make the claim that mixed seating violates Biblical law, the claim that it does is, at best, hyperbolic
It would have made more sense halakhically and textually to claim that mechitza is needed due to the prohibition against saying Shema near an erva. Yet, he allowed mixed torah learning at Maimonides School in Brookline! Like tefillah, torah study may not be done in the presence of either nakedness or excrement. And since classroom furniture is more separate than synagogue pews, there is greater danger of visual impropriety in the classroom than in the sanctuary.
Furthermore, Tosafot and the Mordechai say that a mechitza can be erected on Shabbat for the sake of conventional modesty; yet in Eruvin 94, it is being erected purely because Shmuel wants his privacy.
[Mishnah Eduyyos 2:2]