How to cover exposed attic wall insulation
Plastic isn't recommended in this situation because it would create a second vapor barrier that can trap moisture and result in condensation and mold. Craft paper would have the same problem, this is what is most likely on the other side of the insulation for the vapor barrier you want.

The good solution for this is a house wrap material (often referred to by the popular commercial brand Tyvek) which allows vapor to pass through while blocking water itself on the outside of the home.

You can also put up a thin board, plywood, or even drywall, if the goal is simply to prevent accidental contact. If you already installed plastic, then cutting some slits to vent air both high and low in each wall cavity may prevent any condensation issues.