In 5e, Divine Smite is neither a spell attack nor a reaction, it's a class feature that you can tack onto a successful melee attack.
That said, it is indicative of the kinds of choices that players need to be able to make and currently cannot (or are currently made in a way that is both awkward and doesn't follow 5e rules). Examples off the top of my head include reactions, sneak attack, battle master maneuvers, and divine smite.
