That's not true. Old mercury and sodium lights are much easier to filter since their emission lines are narrow. LED often has a very broad and spiky spectrum, which makes it almost impossible to filter. (Source: astrophotographer). You can buy reliable light suppression filters for sodium/mercury (google CLS or UHC filter), but not LED

Having large LEDs turn completely off and back on quickly may prove too costly/difficult. Having a notch of light frequency missing from every outdoor LED might be a better solution. You could even wear special filtering glasses that would filter out all artificial light.

People actually used to do this. Back when most street lighting came from sodium lamps, you could eliminate light pollution almost completely with a pair of glasses intended for soda-lime glass blowing.

Easier to filter out than incandescents, not easier than the old sodium lamps. Also a single color LED is not too bad. But a (good) white LED has many primary color LEDs and phosphors to broaden the color spectrum as much as possible.

But IMHO, wrt to incandescents they still suck.