Many period tracking apps leak users data.

If you're on Android, get on Fdroid and use Drip. It's simple, does it's job, can give you alerts if you want them, and can be locked/encrypted so the gov isn't demanding a company to tell them when you were possibly pregnant. Also tracks symptoms and body temp and shit for those who want to get pregnant.

Or if on GrapheneOS turn internet off for your period tracker app (but at that point just use FOSS)

ToS never works. Libre software does.