Originally Posted by
davidad
Compressionless housing is necessary for brakes, not index shifting.
Well, the term "compressionless" is misused. Brake housing has to be fairly constant in length but not critically so unless it compresses so much that the brake action is spongy or the lever hits the bars. Properly spiral wound housing works fine.
Shift housing for indexing systems s more properly called "constant length", which it must be to make indexing accurate. Index shift housing has the reinforcing strands nearly parallel with the its length. That keeps the overall length constant as the housing bents with the handlebar rotation or the rear derailleur chainstay-to-derailleur housing section flexes.