Tried to look it up: https://mashable.com/article/samsung-will-let-you-customize-bixby-button

That article claims the option came out when the s10 was out. I seem to remember being able to mod it on my phone as well S9. Can't test it though because while I've had this phone for 5 years now I may have not babied it enough. The physical bixby button has fallen into the phone itself so I can hear ot rattle around. If I were speculating, I would assume it either A. became available to all 3 phones during that same software update, or we remember it sooner being part of something like putting the phone into developer mode so we could have further modifications