what is the slope from the neck to shoulder called

The slope from the neck to the shoulder is usually called the shoulder slope or shoulder line.

In garment fitting and patternmaking, it’s the angle from the base of the neck to the shoulder point. In anatomy or posture contexts, people may also describe that area as the trapezius region, since that muscle runs from the neck across the shoulders.

Quick wording guide

• Shoulder slope : most common term in sewing/patternmaking.

• Shoulder line : a simple descriptive term.

• Trapezius area : anatomical/muscle-based description.

TL;DR: If you mean clothing or body shape, say shoulder slope.