I’m somewhat of an outlier among Whitney climbers in the sense that I am not into alpine starting. In fact, I’ve never started a hike up Whitney before 8:00AM! My last hike (July 12, 2022), I started at 9:15. Usually, it’s starting to get dark on me right about Trail Camp on the way down. So it’s pretty much a given that I’m coming down the crux of the hike (ie the ‘Endless Staircase’ section between Mirror Lake and Trail Camp) in the dark. It’s actually not that bad. You are probably most likely to get off trail in the portion directly below Trail Camp. But there are also a few sections above Mirror Lake where the trail is a little ambiguous. Like others have said, it DOES seem like rocks and branches have been put up to keep you en route on some of the lower sections. But even in the upper sections just below Trail Camp, you aren’t just going to walk off a cliff unless you just aren’t paying attention. You should know pretty quickly if you have gotten off trail. And the best thing to do here is just backtrack.
Another option is to use an app like Gaia whereby you can record your track. This way, you can easily find your way if you get caught on a side trail in the dark. Another thing I should say is that coming downhill is a little trickier in this respect than going uphill. If you end up going UP this section in the dark (as normal people do), you will less likely lose the trail than coming downhill in the dark.