Though the K line provides not an optimized concept with regard to EMC1) it
provides an amazing EMC immunity. (If the hardware is well done, of course!)
1) The K line is a single wire bus with ground relation. So the K line provides only very little protection against coupling in EMC interferences.
In automotive applications the K line is still a common system for diagnosis. As a result
the the K line is also common in EMC testing.
Of course there might be used other diagnosis interfaces for EMC testing, likewise CAN or LIN. There are also ECUs (electronic control units) with an on-board optical interface.