https://www.theverge.com/2019/2/28/18244691/samsung-bixby-button-remapping-update-galaxy-s8-s9-plus-note-8-9

Samsung’s promised Bixby remapping update is now available for owners of older Galaxy flagships, including the S8 and S8 Plus, S9 and S9 Plus, Note 8, and Note 9, after debuting on the soon-to-be-released S10 line.

Assuming you have a compatible device that’s running Samsung’s Android Pie / One Touch UI software, you should be getting a new update that’ll bring the much requested feature to your phone.