Meta data from regular operation, combined with data from apps, and OS telemetry is probably be such a treasure trove that backdooring the baseband processor is probably overkill in most cases, and while I suspect there are phones backdoored in this manner the real threat will not likely be government agencies for most people; it's the risk that non-government actors discovers it and abuses it for criminal purposes.

I suspect various alphabet agencies are perfectly aware of this, and that they're actively avoiding such measures unless they're considered strictly neccesary. After all, imagine the media shit-storm if some North-Korean hacker group managed to start mining bitcoin with 2 billion Android devices and it turns out that NSA put in the back door that allowed it.