[Kevin] doesn’t stock zener diodes anymore. Why? Because for everything he used to use zeners, he now uses TL431 bandgap voltage references. These look like zener diodes but have an extra terminal.

R3 only needs to provide the minimum current for TL431 If the output goes too high then the LED should be off. However the TL431 cathode draws up to a maximum of 1mA even when it would ideally be no ...