Is having characters face the direction they're moving really that big of an ask? I'm no master developer but comparing two X coordinates and picking a left-facing or right-facing sprite seems like the kind of thing computers are good at.
DF has over ten intelligent species, countless animals and other creatures that include oddities like weresloths. Should all of those be drawn in different orientations? There is a lot of variety you could add graphically based on, say, a dwarves' current activity, mental state and so on but you have to draw the line somewhere if the game is supposed to actually release. They did make some other improvements like a less cryptic graphical representation of terrain.
From what I've seen a lot of the development work for the Steam release was about reducing the technical debt that accumulated over the years and now just had to be paid in order to integrate new features and improvements.
Should all of those be drawn in different orientations?
Literally all you need is a single mirroring. Left and right. That's enough so your characters don't spend half their time moonwalking. It looks ridiculous.
Does it bother you that they aren't facing up/north? Why not?
I can't say I've ever heard anyone complain about 'sliding' or 'moonwalking' before. Mostly because they're just disappearing from one coordinate and appearing in the neighbouring one.
For one character, probably not, but if you have 200 dwarfs and a large goblin raid is flooding your gates with about as many units you now have another coupling between the logic and UI that must be tracked bringing your fortress closer to FPS death at earlier stages.