Or understand the difference between software that mines data and software that just does what it says. If it advertises cloud features, then it’s probably a data mining app. There’s firewall apps that make any other app need permission to use the internet at all that you can use as a gatekeeper, but then you need to adopt the mindset of wondering why an app wants internet access instead of just clicking “allow” so that things work asap.

But yeah, pen and paper or even a spreadsheet not dedicated to tracking periods are good options if you want to avoid worrying about all that. Only thing I’ll add is that it applies to a lot more than just period tracking apps and IMO is as useful these days as knowing how to do basic car or home maintenance.