The following node setup is not functioning as a shadeless material.
When the input is set to white, the material renders as grey. You can turn up the emission value, but that does not guarantee that the material colour will render as the same as the input RGB.
Can true shadeless material be created in Cycles/Eevee?
The material is shadeless. However the rendered output isn't 1:1 the RGB value of the emission shader due to color management. If you were to change the View Transform in the Color Management panel to Raw you'd have perfectly white pixels. This is however not recommended for renders where the result should resemble anything like a (virtual) photograph. See the explanation of Troy Sobotka on color management in Blender.
