Button Mapper allows you to remap any action you want to physical or capacitive hardware buttons. This includes volume buttons, Bixby button, and many buttons on game controllers and other hardware you attach to your phone.
You can map shortcuts, apps or actions to single press, double tap and long press for hardware buttons.
The Bixby button on the Galaxy S9 and S9+ is supported as of version 0.62
Bixby remapping of single, double and long press is completely free. No ads. Pro version not required.
Does not require root. If your device is rooted, you will have many more options (essentially unlimited). Also with root, Bixby remapping does not involve any hacks, it is implemented at the input level (native).
This does require an adb command to remap Bixby on unrooted devices. It's a one-time setup. I do not use the method used by other apps because it leads to a poor user experience (i.e. swiping right causes actions to occur, get stuck in a loop)
This app is continually updated with new features and improvements.
Download on XDA Labs: