While many people said she's not the asshole for wanting to go bra-less, she is risking the loss of her job over it which is silly.
"The real question is whether this is a hill that you want to die on. Bras are a pretty standard attire requirement, reasonable for not. As the new person in the office, you do not have credibility to push back on that expectation without serious career repercussions. If people notice you are not wearing a bra, something about your outfit was off.
I say you are NTA, but you need to wear a bra or find some way for it to be unnoticeable that you are bra-free if you want to be able to focus on your work at work," one person said.