I've been told that Snap Mode could be getting removed completely. Indeed, Universal Windows Platform apps are currently completely unable to utilize Snap Mode, and denying UWP apps access to the feature could be evidence enough of its impending removal.
Instead, Xbox looks to be exploring the possibility of a picture-in-picture multitasking implementation, where "snapped" app would simply hover in the corner of your game, rather than wiping out a quarter of your screen with a largely empty bar