I wrote this, so disclosure first.

It measures per-app mobile data against your billing cycle rather than the calendar month, splits what you spent using an app from what the app spent on its own while the screen was off, and blocks apps per network, so something can be dead on mobile data and completely normal on Wi-Fi.

The part I think actually belongs in this community is the permission. There is no INTERNET permission in the manifest. Not a privacy policy promise, it is simply absent, so Android will not let the process open a socket at all.

What interests me is that you do not have to read the source to verify that. One command on the APK prints six lines and INTERNET is not among them:

aapt dump permissions DataSaver-v0.8.apk

The source being public is good for plenty of reasons, but it is not what makes this particular claim checkable, and I think those two things get conflated a lot. Most people who install a FOSS app never read a line of it. A claim you can check from the artifact itself is worth more than one you can only check by trusting that somebody else read the repo.

Under the hood it reads NetworkStatsManager for the figures and uses VpnService in package-list mode with no packet forwarding for the blocking. Both plain AOSP, no Play Services dependency. The only library it pulls is androidx.core-ktx.

GPL-3.0, free, no ads, nothing to buy, no accounts, no analytics, no servers. Android 7 and up.

The honest gap: right now it is only distributed through Play. The F-Droid request has been open at fdroid/rfp#4520 since 8 October, which as far as I can tell is normal queue time rather than anything wrong. Until then it builds from a clean clone.

https://github.com/anubisproductions/data-saver-usage-blocker