In a practical sense, this unfortunately means that once you reach an average motherbase platform rank of 64, the quality of the soldiers you find in the game will suddenly grind to a near halt.
It turns out that, although the game was once supposed to give you those soldiers, they have been disabled from ever spawning in the single player game, forcing you to get involved in online gameplay (and so hopefully spend real money on 'MB coins') if you want to see them. Ever notice how you never retrieve S+ and S++ soldiers in the field while Metal Gear Solid V: The Phantom Pain? You may come across some Boasters that appear to have such high stats, but once you're back in motherbase they will always drop down to S Rank or below.