Overnight, T-Mobile pushed out an update to the Galaxy S8 and Galaxy S8 Plus on its network. It was a somewhat large update, considering it wasn't an OS update, weighing in at around 291MB in size. The update brings the Galaxy S8 up to the June 1st, 2017 security patch, but it does also bring in some functionality to the Galaxy S8 and Galaxy S8 Plus. It allows users to tap on a dot on the left side of the navigation bar and hide those buttons when not in use. Essentially allowing users to use even more of the screen real estate that's provided on the Galaxy S8 and Galaxy S8 Plus.
Once the update is installed, and the user taps on the dot, there is a pop up that explains what is happening there. The dot can be pressed to hide the navigation bar, but Samsung does note that the navigation bar will remain in tact on home screens and such. It always warns that if an app is not behaving correctly with the navigation buttons hidden, that you should lock the buttons in place and then restart the app. But most apps should work properly with the navigation buttons hidden, that's just an extra precaution by Samsung to keep users from freaking out.
It's unclear if this update will be heading to other carrier variants of the unlocked model in the next few days. But it is fairly likely that Samsung is working on an update similar to this for the other models of the Galaxy S8 and Galaxy S8 Plus. The only question is when will it be available. Carriers will need to certify these updates before they are pushed out. Verizon and AT&T have historically had a pretty tough certification process for updates to devices on their networks. Which is actually why updates for Verizon and AT&T variants have typically taken longer to push out, than those for T-Mobile and Sprint models. The unlocked model will likely be last, and that's simply because there aren't too many users with an unlocked Galaxy S8, so Samsung has less resources devoted to it.