While female archers probably haven't been too common in the past, I assume they were fairly common in steppe peoples like Mongols and Turkic groups. Did they have any special equipment or fire with a different tequnique? Or did they just tough it out? I've heard of the mythical Amazon practice of mastectomy, but I assumed that this was a fiction penned by Greeks to further sensationalize the idea of female warriors.
I'd be interested in answers from any other cultures as well, I'm just not aware of how prevalent archery was among women in other areas.
Hopefully a historian will chime in, but I think you probably have a false idea about the need for breast protection. I do archery occasionally for fun and don't wear protective gear - neither do most other women at the range. No toughness required. Sloppy form occasionally causes the string to hit my forearm (which hurts like hell and bruises badly) but never my breast.
Forearm and finger protectors are much, much more common among female archers than breast protectors.