Bikini armors aside, i can't understand why people have an issue with boob plates. Its not the medieval period. There were very few women to fight or wear armor back then. It was a man's thing.
If you want a serious answer to this, "boob plate" simply doesn't work as armor and shows a gross misunderstanding of how armor works.
Here's an accurate explanation:
There's three major reasons for this: firstly, mammary tissue is fat, not muscle, so there's no reason you can't strap them down*; secondly, the last thing you want is a stress point / join where protrusions funnel attacks either into the armpits (one of the easiest and deadliest ways to disable an attacker, there's nerve bundles there) or up into the neck / jaw (again, the neck being a weak spot in armor) and thirdly, it shows a lack of understanding of just how layered plate armor was/is. Think - underwear, padded layers, chain underlay for weak spots (i.e. neck) and then plate on the top. Breasts would be under a thick layer of padding before the chest plate went over the top.
Basically, "boob plate" is worse than "bikinis" - at least bikinis can exist ~ shaped breastplates with separate "boobs" are purely cosmetic, as the breasts don't actually fit inside them, unless you're wearing faux cosmetic costumes.
Lastly, real plate armor doesn't look anything like you imagine it to: https://en.wikipedia.org/wiki/File:Smythearmour2.jpg. It's far more barrel shaped than you imagine.
/thread
*Ask your mother / sister / close female friend about sports bras, binding and how easily bruised they are.
