I'm plonking another bit of 16th century history here, because where else am i going to put it?
“She was strangely attired in a dress of silver cloth, white and crimson…lined with red taffeta. She kept the front of her dress open, and you could see the whole of her bosom, and passing low, and often she would open the front of her dress with her hands, as if she were too hot…”
This is Elizabeth Tudor in her sixties, exposing her flushed yet “white and delicate” flesh like a pagan goddess to the French ambassador, de Maisse. Other visitors to the English court in the 1590’s confirm this ritualised exhibitionism and, after a second audience, de Maisse writes of Elizabeth’s gown being plucked “open even to the navel”.
Elizabeth's breasty exhibitions were fully in tune with fashions of her era and later. Looking at portraits, one can see that such low necklines must have revealed the whole breast, particularly as bodices flattened the norks from below in order to push them up and out as "orbs" above the bodice line. Some portraits include a nipple or both, but it's unclear whether the majority of artists ignored them or they've been painted over by prudish art collectors at a later time.
It's also unclear - as far I can tell - when it was appropriate for a woman to flash her boobage and when she'd wear the higher necklines featured in most portraits. None of these outfits were worn routinely: they were heavy, cumbersome and stiff, only painstakingly donned for portraits, balls and Court occasions. Women - even queens - spent most of their days in softer and looser gear. They would have worn softer, looser bodices too, and it's quite likely that aristocratic households were used to getting an eyeful of breast as everyone went about their duties.
The very deliberate exposure of deep-cut formal gowns and Elizabeth's casual unlacing during diplomatic audiences seem to have been a demonstration of power. There isn't enough information to attempt an analysis of how this worked, but that's what the known events suggest. An earlier PP mentioned power as one possible motive for today's breast-bragging fashions, so they might be channelling the most powerful woman this country (and perhaps the world) has ever known!
One Spanish ambassador to Elizabeth's court was unfazed by her bare breasts but horrified at her teeth, which were rotten and black.