UPDATE: Galaxy S8 teardown reveals an unused OIS mechanism in the selfie camera (but not really)
UPDATE: We received an official response from Samsung, which says that the front-facing camera lacks OIS. The gap that appears to be an OIS system is actually for the auto-focusing lens, since it requires more space than the fixed-focus ones.
It was a well-known fact that Samsung experimented with a large variety of prototypes in order to come up with the Galaxy S8 and Galaxy S8+. Presumably, these featured all kinds of different goodies, both in terms of design and specs. In order to come up with a final product, Samsung had to compromise in some departments, though, just like any other OEM.
Judging by the teardown performed by JerryRigEverything on YouTube, Samsung scrapped a feature that a lot of people would have appreciated. While taking the phone apart, Zack (his real name isn't Jerry) found that the front-facing camera actually has Optical Image Stabilization hardware.
Whether this was something that Samsung wanted from its new selfie snapper, or it was just a coincidence that the chosen hardware had OIS capabilities is still unknown. But it is quite peculiar that the component itself supports OIS, yet this is not mentioned in the specs.
It is also not clear whether Samsung is planning to activate the feature with a software update at some point. In fact, depending on the way the hardware was installed, this could prove harder than pushing a firmware update with the required algorithms for the camera.