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 ...

[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.