It seems that last major development on Niagara was done in August’15 (/EpicGames/UnrealEngine/commit/f7ffe5c68e7c5d0532306f4edc465c76959ed7d5). From then on, only API-compatibility changes were done.
Unless the real development is happening in some separate perforce branch not pushed to master, it seems that Niagara’s development is on halt.