I see no reason why I can't just cast smoky haze backwards, DK has no skills assigned to SHIFT + S.
Otherwise, I have to mouse move it, which is clunky for a dash skill.

I'm pretty sure Pearl Abyss dislike mouse movement anyways, backwards smokey haze should have ideally been a possibility since the release of the class.

As a substitute we have for Awakening: Trap of Vedir and for Succession: Backwards Nocturne

We do also have phantasmal snare and although it's only side to side, it's a pretty damn good mobility skill. 

I honestly doubt they'll do anything like this.

Smokey haze is super armour and is on a much lower cooldown, whereas trap is a rather janky frontal guard skill, moving frontal guards on old classes are usually unreliable.

Backwards nocturne is not succession specific, snare can be used backwards in succession outside side-by-side, I'm not sure what you are saying here.

Regardless, shift + S is still completely free of any skill inputs, there is no reason for them not to do this.