Any idiot and chatgpt could knock up an overt always listening app in an afternoon. I have no doubt shady apps already can do this. Its not hard or expensive. (Backend storage and audio processing costs are a different kettle of fish, and I think those make this fairly prohibitive as well, but that's a funding problem, not a technical problem.)

But as soon as they make the claim that it doesn't trigger the microphone LED on iOS and Android, across all devices, then that's a "technically hard" problem. That's multiple zero days across multiple devices. Its just not feasable for an ad tech firm. They would never be able to recoup that investment.

I'm happy to be proven wrong, but so far all the researchers in the world have found nothing.

So I'm attributing near 0% chance that anyone outside of nation states have the later tech (device agnostic covert audio recording).