"Optional" doesn't mean "exclude it if you feel like it". It means "think about it and decide whether it's worth adding". They can be excluded if doing so doesn't present a danger (e.g. it's a small road).

From the one photograph we've seen of the slip road in question, I'd say the lack of chevrons poses quite a serious danger. It looks just like a normal lane. I've never noticed a slip road branch off like that in the UK, i.e. a big crash barrier up the middle but no chevrons.

(I'm not saying whoever designed this section of road is guilty of some sort of crime, or is somehow responsible for the crashes, I'm just saying it should have been designed better).