The correct name for this multi-bladed weapon is Shuriken and often called a 'throwing star', with between three and eight points; four being common. (See also Star Cross.) There are several styles: smooth blades, arrow heads, barbs, etc. (See also Barby Cross.)
They were never tools of first choice for ninjas, but the name has been borrowed for this implement to make it sound more exotic.
Contemporary stories and movies suggest that ninjas were assassins, when in fact their main job was to gather intelligence, usually by stealth. Their techniques for spying have now been superseded by the internet and whilst they still have a popular dojos for passing on their skills, the two remaining ninja grandmasters (Jinichi Kawakami of the Koka clan and Masaaki Hatsumi of the Togakure clan) agree there is no point in selecting heirs.
The popularity of the Ninja Star remains strong.
Easy to throw, but a bit awkward to catch. They are banned in many countries and a nightmare for veterinary surgeons who treat dogs trained to catch Frisbees.
See also Boomerang
And that's all we have to say about these implements.
As for the Ninja, well, they might seem to be disappearing, but remember their skill at always being ready to change tactic. They could sneak back into the limelight at any time. Their stealth can best be summed up in this little story.