I guess the tabbed washer can make headset adjustment a one-spanner job up to a point, tightening the top cup up, and the locknut down, against the washer in turn, provided your spanner is thin enough to work on the cup without fouling on the locknut. As I understand it, without the washer-tab in the slot, the whole assembly- top cup, washer and locknut- is apt to move around the thread as a unit, both during 'tightening' with one spanner, and perhaps while riding.
Might be misinterpreting you, but, to be clear, I don't see 'two spanners' and 'tab and slot' as two alternative ways of tightening a headset. They are complimentary. Anyway the slot is optional- on some frames it's there and on some it's not, so presumably it's not essential to have one in order that your headset doesn't work loose.
I'd guess the slot in the steerer threads to be original- AFAIK it wouldn't have been common practice to cut one yourself. Yours stops before it reaches the top of the steerer so is done by machine, not by hand. It's much neater than anything I could do with a hacksaw and file. Maybe your headset is not original to the frame, and the original one had a tabbed washer?