Why would anyone want to screen a stepper motor cable?

This reeks of urban legend, a bit like choosing your motor purely on holding torque :heehee:

I'd say lots of square mm for minimum resistance and lots of strands for flex.

I use twisted pair for signal cables and you don't need fancy drivers to reap the low crosstalk benefits. Even humble RS232 at 9600 is good for 100 yards in good old Telecom twisted pair, doubt you'd get 25 with screened.