iNaturalist is an open source app but the Android build depends on a non-free software Crashlytics. I wish there were a 100% open source apk available. Free-software true believers such as me will only install such software, such as can be installed from f-droid.org, which is a libre analog of the Google Play store :)
I understand the sentiment, and while Crashlytics is useful, it’s not great from a privacy standpoint either. One of our goals for this year is to explore ways to reduce our reliance on third party data processors (to use GDPR parlance), which would hopefully have the added benefit of removing this proprietary dependency. That said, I don’t think we’ll ever commit to maintaining current builds on f-droid. There’s just not enough of an audience. I should also say that while we would like to have totally open source dependencies, it’s not something we’re going to commit to. We also depend on the Google Maps SDK, among others, which would probably exclude us from f-droid too.