Quick Answer: Unfortunately, no.
The iPhone 8 and iPhone 6S have quite different specifications.
Question: Will iPhone 8 cases fit the iPhone 6S?
Answer: Firstly, the camera on the iPhone 8 is a different size to the iPhone 6S. Any iPhone 8 cases would not fit around the camera on the iPhone 6S, therefore making them incompatible. The size of both phones might by pretty much the same, but the camera positioning effects how cases fit each model.
Another reason why iPhone 8 cases will not fit the iPhone 6S is because of the different EarPods connections. The iPhone 8 uses EarPods with the lightning connector, whereas the iPhone 6S uses the standard 3.5mm audio jack. This means that the cut-outs in the cases will differ for each model. Although the cut-outs in the cases would not be too different, an iPhone 8 case would still not properly fit an iPhone 6S; it would look stretched and warped.
You may ask why something this small, like the camera being larger or the cut-outs being in different places, would affect the fitting of a case. But it’s things as small as these that mean an iPhone 8 case will not fit an iPhone 6S.
So, will iPhone 8 cases fit the iPhone 6S? The quick and simple answer is no.