Manufacturing of Translucent LED Indicating Symbols (e.g. router indicator lights)
Based on my observations of these physical products, my best guess is that they are using some type of translucent material, possibly tinted acrylic, and using some method to print the negative symbol shape on the back side of the acrylic. Then a light pipe or direct LED lighting can be used to illuminate each symbol.
Could anyone confirm if this is the method that is used? If so does anyone have more details on specific materials, printing processes, and manufacturers which can provide this service?

Yes, the light is fed in through a tube and the area where the light is desired to be emitted is clear, while the area where light is not desired to emit is opaque.