Team,
I have a stereo where I have a 12V 100mA pin that is ON when the device is powered on and to 0 when the device is off or standby.
Is there any component that can detect that and I can read from HomeAssistant to trigger some scenes?
You could use a hacked door/window sensor and a relay. Power the coil of the relay from the 12V out of the stereo then wire the relay contact in place of the door/window reed switch.
Create an ESPhome node using a device of your choosing (Wemos D1 mini or whatever) then use a resistive voltage divider to then give a 3.3V input to a pin on the ESP. The ESP would need power for this option rather than the above battery option.
There would be other ways as well, just need to be creative.
Following the route of ready to use components you could use Fibaro Smart Implant (for Zwave) or Shelly Uni (for WiFi). Both can be configured to use their inputs as analog voltage meters so detection should be easy. Perhaps only some step down resistor to be used as input voltage is limited to 10V (at least for Fibaro).
Both of these, similarly to ESP solution that @spakydave advised, would require some permanent power source.