If you have an Apple computer and are willing to download some code there’s period underground… it’s terribly written by me for my wife. She uses it on her iPhone and it keeps all data local. I never tried to get it on the App Store due to qt licenses, and because the encryption module didn’t compile so it will rely on you not unlocking your device for someone who shouldn’t be seeing the data. There’s other options I’ve seen on here hopefully one of those will work.

It does exist on the android store with encryption as an option and blank data with no warning if you enter the wrong password. Also includes a quick delete and a quick delete with random data written into the database as deletion options

Pretty unlikely that I ever port to iPhone, I dont have one to dev on. To be honest, my partner doesnt even use it, so it was a little bit of a waste of time, except for a learning opportunity.

Given that Apple is more likely to submit to government (any, any country) coercion and doesn't allow side loading of apps your next phone should be Android. Especially one that supports GrapheneOS or CalyxOS.

Failing that, many apps work as web applications and if saved to the home screen it'll both look and feel like a native app.