yitayningwut -“Perhaps the “minhag” of these segments of Klal Yisroel came about because they hold that the women never accepted upon themselves to fast”
Sorry the Rema says Befairush and the MB explains -this is only Pregnant and nursing women who didn’t accept it upon themselves. On e/o else including other women the Biur Halacha says -“it became a Taanis Chova”. And like Sam2 says -this is “Poshut P’shat” in the S’A. So you can’t say there is a Minhag for certain segments not to fast except for pregnant and nursing, unless you find Achronim whom argue on the S’A and then you can be Someach on them. Right now all Poskim can do is say that the certain individual fits into one of the categories were they don’t have to fast; eg. -sick, pregnant or nursing. I just think the Brisker Rov & others held most women will fall into one of these categories. But nowadays we have medical tests to know if s/o is pregnant or anemic or something like this. So I can’t see anyone just giving a blanket Heter for any group not to fast!