When I use a graduated style and I set the outline of the polygons to transparent I can still see the border. If I set the width to "Hairline" and/or the style to "No pen" I can still see them.

I have found multiple similar questions with very manual intensive solutions that are simply not practical in my case:

This seems to be an old feature request, am I missing something?

I'm using QGIS 2.18.