NeoPixels are prolific, used in countless hobby, industrial, and commercials products. They have become synonymous with "RGB LED". There a several variants but the datasheets all specify an operating voltage of 5V, more specifically, 3.5V to 5.5V.
Nevertheless, I have seen many designs power NeoPixels from 3.3V. I do not do this myself and will always provide a 5V supply, using a charge pump or similar if necessary. It just seems unacceptable to me to end up in a situation where a product I have designed starts misbehaving because I ignored clear engineering data.
I am also not satisfied assuming that empirical data overrides datasheet specifications. Just because the first ten thousand units work, it does not mean that the next ten thousand will. Manufacturers change components characteristics knowingly or unknowingly and only the datasheet specification can be relied upon.
So my question is, can NeoPixel 3.3V operation be verified analytically, such that a design engineer can take an informed risk?
For example, perhaps it is known that the logic is 3.3V compatible CMOS but the LED Vf is 3.5V which would mean reliable operation at 3.3V but with compromised brightness / colour mixing.