Source? AFAIK they only have hardware indicators for webcams on cams, and it's not used for microphones.
They started in ios14, iOS 17 got new Secure Exclave path that (A18, M4).
Search for “Secure Indicator Light”.
Also searching for “Secure Exclave” will reveal some fun reads.
https://9to5mac.com/2024/09/11/iphone-16-privacy-feature-m4-...
I couldn’t find a more technical article but this was when it was released.