Edit: Sorry, seems I was wrong [1] but you can host your own F-Droid repository which can be added to F-Droid by users to install your app from.
https://forum.f-droid.org/t/non-open-source-apps-are-allowed...
What made me wonder about it is that this is very specific wording that indicates that they proactively know the author is lying, when it would be very easy to instead say something along the lines of what you said, that it is too hard to verify without access to the source code.
I think it is important to be specific, clear, and to have evidence if one wants to call somebody a liar, though.
Or maybe it is something else, it could be interesting if they have some other definition of “privacy respecting” that precludes closed source apps, for example. That is, to “respect privacy” could be understood to actually be to provide users with verifiable evidence that their private info isn’t compromised. I think this isn’t the conventional definition definition of privacy respecting but I’m definitely ready to be pulled on-side if anybody starts pushing it.
Plus if there is one legitimate network call, then this strategy is out since you can't know what that request contains. OP using in-app purchases, so I'm willing to be there's at least one network call in there.
If there is no network access permission at all, then I think we agree, that's a reasonable guarantee.
Syntax error: “Google” and “safe” used in one sentence.
If the user is already using a Google Account for the app (you need a google account to officially download apps from the Play store), they are okay with minimal use of the Google account.
Google might be evil in its ways but the google app data is far different than sharing your personal data with startups or other apps who use your data for other purposes when saved on their servers.
AFAIK, the google appdata does is like a data dump on the user's own google drive, its private to the user, and not used by Google or shared with anyone else, it only can be used and accessed by the app which saves it. Please do correct me if I am wrong about this.
I'm using an unofficial Google Play client, Aurora Store, which can use anonymous accounts instead of your own account.
Anyway, giving the user an option to backup data in Google is better than no option at all, although I'd also add a simple export to file, too.
Then you certainly shouldn't be using Google (or any third party service that isn't encrypting customer data in an end-to-end way) for this, as Google is subject to law enforcement requests and right wing states have already tried to use this to access healthcare data. The whole point of this app seems to be to prevent that.
Most women are increasingly desperate to be the victim of something and "not being able to track my cycle because white male Republicans want to control my body" is a super popular go to right now.
Since this is on Google Play Store I want to ask why is it not viable with user's consent to just sync app data with your own(user's) Google account?
I understand (and support) the reluctance to register and save data in some third party server which likely will use or sell your data but isn't app data in your Google Account very safe?
Google App Backup is basically storing user data in their own Google Drive Account, and is more like a sqlite dump isn't it? https://developer.android.com/guide/topics/data/backup
And as many suggested, you should consider listing it on F-Droid too if your app requires no account at all.
Good luck!