Yes Sir, this is the feature in question. I used what is possibly a work around - but it functions perfectly for me.
Background.
I used a Samsung Galaxy S6+ for my initial development on my app and set the derivative screen size to 411 x 731 x 1.
I then got the Samsung S8+ with the screen feature mentioned above, whose size in real format is 1440 x 2792 x 3.5 - if you enter this as a derivative it normalizes to 411 x 797 x 1. (Note only the difference on the height)
OK. So testing this app on the S8+, I did get the black stripe at the bottom of the screen after double tapping the little pin dot on the bottom left of the screen whilst in the app.
Work Around.
There are two ways in which you can go about it (as I did - adapted to the requirements of the display I was building).
1. Add a new derivative of 411 x 797 x 1 and adjust your design to suit, to fill the screen appropriately, or
2. I used anchors where I could to secure both the horizontal and the vertical sides in the Designer. This solution eliminates the requirement for adding the new derivative and will automatically adjust your display size depending on what the app user has set their desire to use on their Nav Bar.
The only thing I have as yet not figured out, is how to adjust the app screen immediately when the user taps this pin button. Currently I have to go one page back and then re-load the page in question - it then adjusts to fill the screen, and implies that you need to re-start the app if this happens on the Main Screen. I am currently checking the IME solution and will report back if I get it to work.
This will probably also be a requirement in b4i soon as I see the new iPhone X has the same feature.
One last word of advice - watch your designs for these new screens - If you use a "square" icon (especially on full screen apps) in one of the corners, it will most likely be rounded and cut off as this is what these screens do - they round their corners.
I trust that this will help in the meanwhile.