Was there a first spacecraft put in a polar orbit?
It might not be clear if it's too hard to say what high inclination orbit is or is not polar, for example if 91 satellites were launched in 0, 1, 2... and finally 90 degree orbits in sequence.
But if there were several high inclination spacecraft used by unnamed northern hemisphere countries to look at each other or in Molniya orbits and then suddenly there was a definitively first sun-synchronous orbit, then that might be it. A quick check of that article shows no instance of the word "first".