That's a good question, headshots are always a tough choice, because we have to spend a few minutes finding proof that she's actually in clothes and not naked or whatever. We enforce the formal rule here because otherwise people would just crop out anything that wouldn't be allowed, this happens a lot.
In regards to that Sophie image, i had to go find something that showed that she was in something appropriate, i found this image. So i approved the post.
I assume that means the formal attire must be visible, not assumed?
Preferably, yes. But it's not always the case. Some photoshoots feature a lot of headshots but also have other images that show the full outfit, if you post an image like that, it helps us if you include the rest of the images in the comments, that way we can approve the post. But you should also know that we don't just straight up remove all headshots because it might not be formal. Only when we find proof that it's not. Like earlier today, this post was approved by another mod, but it was later reported and i found this other image that showed that it wasn't really a formal outfit she was wearing.
Also, the lady could be wearing casual clothing even if the hair styling/makeup is formal, another point to consider if you are only taking into account the head styling.
As you can see in that post i referred to earlier, this was exactly the case. Formal hair and makeup (according to me) but the outfit, not so much. We can't confirm every single headshot, some we have to approve because there is no other image to show that it's not formal.
I hope that answers your questions, if you have anything else you want to know, feel free to ask away.