I was told by a man that should have known, that the split lock washers not only did not prevent loosening as well as proper torque and a thread lock material, OR a self locking nut, be it nylock or crimped metal, but in testing was always the failure point in high strength fastener testing. No matter how good you make them, they always fail first due to the fact that they're not a solid ring. The split lock washer was part of the world of World War One, where most of the bolts were barely better than mild steel and had square heads. I still use them on grade five bolts if its somebody's garden tractor or utility trailer, anything where it's a critical fastening point, or I'm building it for sale, I used nylocks, or on blind threaded holes, Loctite.
As to the split pin issue, just as Tony sez, if there's a place for them, I put them in, more to give it an authentic look than anything, or to avoid some old-timer from saying I was lazy and left them out!
