• The default Geometry renderer still renders it, but as it is so thin, it appears as only a few disconnected pixels here and there (visually useless, see screenshot).
• The Curve renderer disables the stroke, nothing is rendered.
• The SW renderer still renders it, and sets the QPen width to 0, which triggers the old magic that makes the actual width 1 and the pen cosmetic.

Attached screenshot shows the effect on some overlapping shapes. The stroke has width 0, color white. The red shapes are rendered by Geometry renderer, the blue are the same rendered with Curve renderer.