In the O.T. God walked in the garden with his creations fashioned in his image. Sin separated the relationship and the intimacy of the garden. Not until Christ, walking once more with the humans, is the way to restore that original intimacy made more visible. Jesus proclaimed in his opening salvo announcing his ministry that he was a fulfillment of the prophet Isaiah's words and was preaching hope to the hopeless, powerless, and outcast.
This perfectly expressed the state of woman in the N.T. To the Greeks she was often sub-human, to the Romans a necessary evil, and to the Jew a means to an end (the path of the eventual Messiah). She was a commodity to be bought, sold, used, ignored, and discarded. She could testify in court, often could do not business, and faced daily the limitations of being a woman in a "man's world".
Throughout the gospels one woman is found following Jesus with great dedication, she is at the cross, and she is at the tomb. The definition of the term 'apostle' is one sent with a mission. So Mary Magdalene, when Jesus told her to "go and tell"...became the first apostle.
So, by the time that Paul in Romans is calling Junia an apostle....it is not a new thing....but it has, apparently over the centuries, become the forgotten thing.