I read somewhere that proper oral posture/mewing/fagga will give you a more masculine brow ridge and lower your harline. Like, it makes your face more masculine (if you're a man). Thoughts?
I haven't heard anything about brow development. Growth of that bone is tied to testosterone levels. This is why women don't develop brow bones. Actually in men and women the forehead may actually get flatter as many have discussed here prior. Not that your brow would go away but it certainly would not be more emphasized.
What can happen is the eyes get deeper set (more supported). This has happened to many people.
Hairline lowering is unverified at this moment .
I will say proper posture makes your lower third stronger if it was previously deficient however. That has happened to me.
If a 25 year old has low testosterone and gets testosterone replacement therapy, will their face become more masculine?
The testosterone will result in more muscle mass meaning things like your jaw muscle increasing in size. It will also likely result in less body fat which makes the face leaner and thus changes its appearance.
Trans who try to transition from F to M by taking test also experience changes in their bone structure. I believe that this is a very slow progress and takes several years at least for noticeable results but don't quote me on it.