text: Get rid of Navigation Bar keyboard button. - Android Forums at AndroidCentral.com
  https://forums.androidcentral.com/showthread.php?t=822220&s=6aeaf35087349627d5d7abd11b4fdb54&p=5910612&viewfull=1#post5910612
  Only got my S8 yesterday and pretty much installed Smart Keyboard Pro immediately. From today for some reason I am now getting a little keyboard icon on the far right. It is giving me a choice of Google KB, Smart KB or Samsung KB. It's annoying me as it's where the back arrow used to be. Now to the left of the KB icon there is a down arrow I have to press and then the back arrow.
  This wasn't there yesterday and I was using Smart KB all day. So as much as I think it's to do with using another KB I think it must a setting that's changed somewhere.
  https://forums.androidcentral.com/showthread.php?t=822220&s=6aeaf35087349627d5d7abd11b4fdb54&p=5910643&viewfull=1#post5910643
  The Keyboard selector button is baked in and I don't believe that can be changed. it is a feature when you install additional keyboards, giving you a way of switching keyboards as you're typing. There are several purpose built keyboards for specific functions. For example, I have a "keyboard" installed where literally the only thing it does is let me insert ¯\_(ツ)_/¯ into text.
  The down arrow you see appears any time you have a keyboard open, it doesn't matter if you have 3rd party keyboards installed or not. That button serves as a visual UI/UX element to show users how to hide the software keyboard when it is displayed on the screen.