The latest Galaxy S8 and Galaxy S8+ update on T-Mobile seems to be blocking Bixby remapping applications, essentially disabling any possibility to make the Bixby button do mostly whatever you want it to do. For some this won't be an issue but for other users it could certainly pose a problem, or at the very least an annoyance if they've been used to using these types of apps to more or less unlock the Bixby button's potential.
This is reportedly only an issue with T-Mobile models of the Galaxy S8 and Galaxy S8+, at least for now, as there doesn't seem to be complaints from users or developers of these types of apps in regards to the Galaxy S8 and Galaxy S8+ from other carriers. It's also worth noting that this isn't something which T-Mobile itself would be putting in place and that the blocking of Bixby remapping apps is something which would have been issued by Samsung. This also wouldn't be the first time that Samsung sent out an update which disabled these apps from working, so it might not be much of a surprise that the apps have been blocked again after they started working for a short period of time following the first time they were blocked.
Samsung could be blocking these because it wants the Bixby button to be for launching Bixby only, but the reason these apps are here in the first place is because Samsung didn't design the button to allow for remapping so it could launch anything the user wants. Though the workaround for the button remapping has been patched up by Samsung for now, it's not impossible for another workaround to be patched in by the devs who are making these apps, so users who enjoy them should keep an eye out for any updates to them as a fix could end up becoming available at some point in the future. If you're a user on T-Mobile and have the Galaxy S8 or Galaxy S8+, the fix from Samsung that blocks the button remapping apps again is part of the June Security Patch update, so if you want to avoid the apps from being disabled you could simply choose to not update the device, though that likely isn't a recommended course of action given that the update has to do with device security.