Doesn't have to be particularly soft. Coin silver wire (2mm to fit standard draughting leadholders) is "normal" for the silverpoint drawing technique, and there

Doesn't have to be particularly soft. Coin silver wire (2mm to fit standard draughting leadholders) is "normal" for the silverpoint drawing technique, and there wouldn't be a whole lot of sharpening involved if super-fine cross-hatching wasn't one of the expectations ("scumbling" a duller wire will also achieve shading).