That is cool but now you're relying on some random third-party app developer for your security/privacy. Even if they're good people, there's nothing to stop them from selling it to some jerk that will enable their own apps to bypass it and/or enable other shady developers to bypass it for money.
This kind of software can also be difficult to get right. There may be ways to easily bypass it.
If Google did this, I'd seriously consider going back to Android.
https://play.google.com/store/apps/details?id=app.greyshirts...
For example it helped me find a trustworthy e-mail client (K-mail) by only allowing network access to my email server.