There is concern that spline wear can lead to aft migration of the Aft Shaft, so indeed the Splines DO attenuate Axial movement
The AD you refer to does explain what would happen if the splines wore through. In practice, no flight engine is allowed to approach that limit. If the splines were worn through the half shafts would rotate relative to one another and undo the axial load coupling (a threaded component most likely). But the splines thenselves only transmit torsional loads - except that the helical shape may serve to unload the axial coupling in some circumstances.
The QF32 related AD's are not linked to the spline wear AD.