Your second link for swiftkey is broken but I see what you mean. I think it's a limitation of the stock keyboard. With the newer models like the Note 9 and S10s they instituted software to make one-handed use easier. I think that's what is happening here is that because of the software, it makes this huge gap at the bottom. If it's fixed through a 3rd party keyboard I honestly recommend using one and I'll put in a feature request for the stock keyboard.
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One reason for this may be that going from gestures to buttons, your typing position would change, and there is no chin on the bottom of the phone to comfortably hold the phone when typing if the keyboard drops low. This is exactly why Apple does not lower the keyboard on the iPhone X series of the phones either.
It seems like a possible idea, however, I don't have any inconvenience when reaching the bottom of the screen while using any other app. If this is still the case, it should be customizable, considering there is already a floating keyboard option..
