Google still decides how Android is developed. Also you will never be able to run mainline Linux kernel on those devices due to the proprietary drivers. If you really want to escape from Google and own your phone, consider GNU/Linux phones instead.
Every flip phone since 1997 has had apps.
Internet
Sure they do. I used to develop web sites for flip phones.
or sensors
Well, isn't that the point of a flip phone these days? That you don't want to be constantly sensed, tracked, and tabulated? Sounds like a feature, not a bug.
Also, they don't have kill switches
There's no better kill switch than the ability to pop out the battery.
If you really need to be sure, you can also hire a professional and they will verify that for you, too. Try to do that with iPhone software and hardware.