According to https://source.android.com/docs/core/permissions/privacy-ind..., the microphone indicator is only in there since Android 12. Android 12 and 13 cover only 50% of Android phones, according to https://gs.statcounter.com/os-version-market-share/android/m.... There were some "access to the microphone is restricted for background apps" changes earlier, reported for Android 9. But I wouldn't rely on them, and even if those restriction always worked, that still made ~10% of Android phones vulnerable.
It seems highly inefficient to listen to users 24/7 given the other more specific signals that are available. Rather have a transaction data point around everything someone has purchased then what they talk about.
