What is blue light shield on smartphones?

It's a filter that blocks blue light emissions from the display. You could achieve the same effect with software, by turning up the color temperature. Blue light keeps you awake and can cause eye strain.

it ups the yellow factor in your screen to offset the blue light that will make you stay up and not blink staring at your phone/Tablet/PC screen

I don't doubt the explanations but why not simply be more discriminatory in using the instrument, even switching it off when not needed?

allows you to flexibly adjust the amount of harmful blue light emitted by your screen to lessen the strain on your eyes as you read and view content

The blue end of the light spectrum is most harmful to the human eye.

The color blue, when illuminated, resets our circada rythem and causes insomnia.