In the time before mirrors we may not have see our physical selves but for the fleeting reflections from calm bodies of water.
Like a blind person who's hearing becomes more sensitive, or a def person who's eyes eventually learn to 'hear', how was our perception of self and others different than today?
From what mental estimate did a pre-mirror age self-portrait artist paint? Furthermore, when they finished how would they have know if it resembled them? There is no metric for truth here. It seems strange to rely on the fact that only other people could identify you as you.
The person that you see in the mirror or in a photograph may not be the person that other people see. But how are we to know? Artists brave enough to paint themselves over and over again are evidence that what we see in the mirror may be little more that what we imagine we are seeing. A mirror is a mirror and yet our 4th grade self-portrait is different to our 12th grade one and still different to our grad school version. Skills aside, same person, different mirror.
For some it may not be a mirror discussion so much as one of medium. The medium of the self-portrait is one that cleverly side steps some judgement because it's so personal. For example, Frieda Kahlo showed the many sides in every person by showing them in herself. Perhaps it freed her from the inevitable criticism of holding a mirror up to others for whom their reflections were not something they want to see.