Does a modern MacBook's green "camera light" always turn on when the camera is activated?
There must have been a group of designers and engineers who created the camera and camera light system – they would surely know and have told others.


John Gruber received an email from one such former Apple engineer:

https://daringfireball.net/2019/02/on_covering_webcams


  Security researchers at Johns Hopkins released a paper in 2013 revealing that the indicator lights on Macs released prior to 2008 could be circumvented by software. I linked to this in 2016, wondering if the same exploit was possible on more recent Macs. Here’s an answer I received from a former engineer at Apple who was intimately familiar with the software drivers for Mac webcams:
  
  
    The original cameras had the problem that the JHU researchers detailed in the article that your linked to. Problem was that the firmware was downloaded on every boot and there was no security/encryption mechanism for verifying it. The part used was fairly common and the firmware was just in RAM (hence the loading after a cold boot), as oppose to flashed.
    
    All cameras after that one were different: The hardware team tied the LED to a hardware signal from the sensor: If the (I believe) vertical sync was active, the LED would light up. There is NO firmware control to disable/enable the LED. The actual firmware is indeed flashable, but the part is not a generic part and there are mechanisms in place to verify the image being flashed. […]
    
    So, no, I don’t believe that malware could be installed to enable the camera without lighting the LED. My concern would be a situation where a frame is captured so the LED is lit only for a very brief period of time.
  
  
  The still photo problem — where the light only turns on for the instant the image is being captured — is interesting. But I would wager real money that the camera indicator light cannot be circumvented by software on any Mac released this decade.


emphasis mine