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Period tracking app refuses to disclose data to American authorities
(www.newsweek.com)
This is a most excellent place for technology news and articles.
Something being FOSS doesn't necessarily mean it's safe / ethical, but a LOT of FOSS apps are designed with those principles in mind.
However, being FOSS means that if an app claims that it is safe / ethical (ex. In this case, not storing data anywhere but on your device), you or an experienced peer can check the code to verify that fact.