That would be nice, and I love the camera LEDs. Also, it is far harder to physically stop a microphone from receiving audio as compared to just slapping something on your camera. Physical barriers are important, as software cannot bypass that.
There are things like Mic-Lock[1] but software could be written to bypass it and use the built-in microphone (still love these though!)
While not a hardware LED you could use Oversight[2] or Micro Snitch [3] on Mac. Oversight users free, but Micro Snitch comes with Little Snitch. I use Micro Snitch, so I do not have much experience with Oversight, but I love how you can set Micro Snitch to have a floating icon in any corner you choose. This icon would turn on when either your camera or mic is activated. Therefore, you could see when it is turned on. If it gets annoying, you can have it in the menu bar as well.
While it is still a software solution, it’s the best we have. Hopefully one day we can have a microphone LED.
[1]: https://mic-lock.com
[2]: https://www.objective-see.com/products/oversight.html
[3]: https://obdev.at/products/microsnitch/index.html