This feature was originally developed for AMOLED screen devices, which is determined by the illumination principle of different screens.
Some ROMs of some LCD screen devices have this feature, probably because of the needs of some users, but this is not necessarily good for the device.
Each pixel of the AMOLED screen is shine separately, which means that the AMOLED screen will display almost no battery consumption when it is black.
But if it is an LCD screen, the principle is completely different.The LCD’s light comes from a row of LED lights at the bottom of the screen, its color display is achieved by controlling the refraction and filtering of light. Which means that the always on display of the LCD screen is not true black screen, it will bring about a relatively larger battery consumption like the bright screen.
So in my opinion, LCD devices don’t have this feature, which is normal.