I have tested quite a bit in a handful of different areas, used single and dual Daggers, and I am not getting half kill to appear. I've used heartbreaker, chain blow, and blood stab to test it.
I see critical, I see overhit, I never see half kill or lethal every referenced. Maybe half kill is a pvp only thing now? I have not tested pvp yet, I didn't look for it during oly.
@Juji can you take a look at this? I wonder if yul is seeing the same...