The funny thing is, I just saw again a documentary about beauty from 2013, so several years back and I know there were also articles on this around at least here in Germany - they basically were going on about scientists trying to find what makes womens faces beautiful and it was mentioned that a symmetric face was important, smooth skin was important and basically the best is a combination of baby face traits with mature female traits. Mature female trait is for example high cheekbones, considered rather beautiful at least in the West where people are looking for young adult women as beauty ideal, not the preferrence for child-women as it is common as a beauty ideal in parts of Asia. And interestingly they also said a high but rounded hairline is ver beautiful female - apparently this is also somehow more one of the baby-face traits - together with larger eyes.
So I am not sure where that "low hairline = female" comes from , really. Obviously if you have a receding hairline and clearly hair loss or baldness its a different thing, Also obviously if the shape is not rounded anymore but square.
My personal theory is the Alpha-Omega theory 😀 - The typical male hairline is that of the "alpha males" - strong widows corners, very square or M shaped, symbolizes strong masculinity (gets weakened when the hair loss reaces the back of the head or center part of the hairline, then it looks old). The opposite is the Omega-Hairline, shaped like a greek Omega Symbol Ω - I almost misgendered a man because of it once because its something almost exclusively seen in women. The center of the hairline is high, actually, but the sides are lower - so in a way its "inverse widows corners"... so its a bit of an expansion of the VFFS concept in that the center hairline is indeed the same in all cases except the ageing man. but the hairline at the sides can be rounded - which is neutral or female , usually considered female then, square or even M shaped, usually considered masculine or Omega shaped, making it a more stronger female gender marker if the rest of the face matches it. I saw this especially in Indian and Polynesian women though, so I am not sure how well this applies to caucasians - it seems for caucasians a high at the center but oval hairline is feminine. For women of african descent, it seems an overall high but perfectly rounded hairline is favourable and culturally often enhanced with braids, exposing the hairline and forehead.
Did you know that some women actually shave the center of the hairline to make it move up? Weird, huh? I tried that once as well, experimenting a bit on myself - since it made my hairline rounder instead of that Mc-D M shape, it feminized my face overall to a degree. Which was very interesting. It helped me decide to get a coronal FFS incision for sure, no hairline incision.