I know it would not be practical to upgrade an S40 to ABS, nor any other bike, either. Drum brakes themselves could be made to work, but they would have to be hydraulically operated instead of mechanically. The semi I drove back in 1999 - 2000 had ABS, and it used drum-type air brakes. I don't know the exact dates, but semis have been required to have ABS since about 1999, and school buses since about 2000. My question about an ABS bike was more rhetorical, in that ABS has been available on at least some bikes for a decade or so. Those of us - including me - who purchase bikes without ABS go into the thing knowing we may be less safe, and that injudicious use of brakes may cause a fall and/or a crash.

Edited to add: My 1994 Pontiac Sunbird has drum rear brakes, and the four-wheel ABS works nicely with them.