This is according to a post found over at XDA Developers, which indicates that an over-the-air (OTA) update has been pushed out to devices that disables button remapping for Bixby. Considering that this remapping behavior was enabled initially, there has to be a reason why Samsung changed its mind on the issue.
Well, according to Samsung, it closed down the remapping function because it was in essence a system exploit that developers discovered:
Samsung was no doubt concerned because apps like "All in one Gestures" took advantage of Accessibility Services in order to reprogram the Bixby button to launch any application of your choosing. The process was a bit long and convoluted at times, but once complete, you could, for instance, remap your Bixby button to launch the Google Assistant.
Samsung’s solution was simply to prevent the Accessibility Services from “intercepting” key events, which is what made remapping possible. For now, it appears that Samsung doesn’t look too fondly on customers bypassing its nascent digital assistant, but the above tweet at least leaves the door slightly open. And from the Samsung’s perspective, it actually wants customers to embrace Bixby, because the company actually thinks that it provides a better user experience.
However, given that Bixby will launch in a somewhat “unfinished” state, it’s also understandable that enthusiasts would like to use the button for other functions.