"From actual use Shimano found that brake style housing did not have quite enough resistance to compressing as they needed for SIS shifting. Hence the development of the stiffer "compressionless" shifter housing that we all use these days."

A bit of an understatement, perhaps.

With age, the outer plastic sheathing can literally "go away", leaving nothing but a bundle of strands, not enough to even apply the brake lightly!

Jagwire sells brake cable kits which include compressionless housing now.
The trick is in the outer covering, it appears to be made of woven stainless steel.
I'm looking foreward to the day that I can buy a bulk roll of this housing by itself!