In many fantasy stories, female warriors wear "boob plates," or armor that is shaped around their breasts. In real life, what did female warriors in medieval times wear while fighting?

We have very few records of women wearing armour, let alone fighting in it, during the Middle Ages. Sikelgaita, for instance, allegedly wore armour and commanded one of the battles during the Battle of Dyrrhachium in 1081, Joanna of Flanders allegedly wore armour and led a major sally during the Siege of Hennebont, and Joan of Arc had armour made for her.

Sikelgaita lived during a time when plate armour was non-existent, so she couldn't have worn boob plate, and it's unlikely she would have had her breasts emphasised in any case. Not only do you want as little movement in your breasts as possible when being active, but the layers of padding and the mail over them would have de-emphasised her bust in any case.

Joanna of Flanders is an interesting case in that she is alleged to have actively participated in fighting twice: once on her sally from Hennebont and then again when she returned to break back into the city. Froissart mentions her fighting a third time onboard a ship, but this is a story I take with a grain of salt. In any case, as a middle aged woman with no prior recorded experience with fighting, I don't think it's likely that she had custom armour and was probably wearing mail or a borrowed coat of plates from a wealthy squire or short knight. This would again preclude boob plate.

Finally we have Joan of Arc. While she probably never actually fought, she did have armour made for her, and near contemporary depictions of her in armour show it quite clearly as being of the same kind used by men.

As to the practicality of boob plate, it's not going to be very practical. Armour tends to bulge out from the chest in a curve in order to keep the metal at a distance from the skin to reduce the chance of injury when an attack gets through your guard, as well as to deflect blows to as great a degree as possible. The less energy absorbed by the plate, the better it was for the wearer. Boob plate would not only transfer more energy through the metal than a proper breast plate, but you also redirect and trap some attacks so that they hit full center in the wearer's sternum, drastically increasing the chance of injury.

There are other considerations as well. Those women who wore armour in command roles were a rarity, women performing a man's role in a time when there was a lot of misogynistic rhetoric from both secular and ecclesiastical authors. By emphasising their feminity, those women would have only drawn attention to their alien presence on the battlefield and likely undermined whatever authority they had claimed through force of will.

Finally, there probably was a small subsection of women who fought disguised as men. I'm not aware of any medieval records of this, but with 17th century examples such as Catalina de Erauso and many later women who were extremely successful at passing themselves off as men for decades, I think it would be foolish to dismiss the possibility of some few women in medieval times fighting as men-at-arms of foot soldiers. These women, of course, would not have worn boob armour.

Fake edit: also, yes, armour would need to be fitted differently for a woman than a man, but plate armour was generally custom fitted in any case, so that would have posed no problem.

Real Edit: Tagging in /u/TheLordHighExecu and /u/sunagainstgold: I found a drawing of Isabell of Castile's armour while looking her up. I think you might be both interested.