Don't want it to hide, want it to be transparent.
Till the previous update you could set it to black however they changed the colors now I guess to avoid screen burn in.
I think it's not very easy and neither Samsung cares what few customers want (Apple never did). App development on Android is not as tightly regulated by guidelines as on iOS. The Android framework is mainly used for OS layer while UI is left to OEM. This is creating a big contrast among experiences on different smartphones. And the Elephant in the room is UI scaling which android has yet to address effectively.
App development requires consistency which unfortunately Android lacks unless an OEM leaves UI untouched (OnePlus, Google Pixel). The difficult task is to integrate hardware with software layer. If, for example we install AOSP ROM on S8, most of it's function will not work. Because the libraries required for proper usage of hardware are property of Samsung.
So with 18.5:9 aspect ratio, what you see is what you get. The screen ratio is unique to S8 and G6 which do not make a large part of android market, so App developers don't care. Unless they are on a big budget. It's up to Samsung to provide software tweaks for us to enjoy Apps not optimized for the display (They have already provided a fullscreen hack). The good news is Samsung has much better development pipeline now a days, may be hiring better designers which may be in future provide a more robust user experience.
This is to say in other words, S8 is too far ahead of current generation of Android hardware market.
PS: If Apple releases an iPhone with OLED display and better specs, I will switch in a blink because of quality of Apps, not the hardware. S8 however is purely an hardware experience, not a fan of android myself but it's good enough.