Your notes stay where you keep them
Renskrift reads and writes only the files and folders you open in it, on your device or in the cloud storage you already use. It does not copy them anywhere else, with one exception that stays on your device.
History is kept on this device, and nowhere else
So that you can go back to an earlier version of a note, Renskrift keeps earlier states of the notes you open in its own storage on your device: when you open or close a note, about once a minute while you work, when another app changes it, before you decide about changes made on disk, and before and after you restore an earlier state. They are never sent anywhere, never put beside your note or in its folder, and not included in your device's backups.
File ▸ History… shows them. You can clear one note's history or all of it there, and turn Keep History off (View ▸ Keep History on the Mac, the Writing menu on iPhone and iPad), which stops it and offers to clear what was kept. Older states are removed on their own as a note's history grows, and when all the history together passes its limit, the whole history of the notes you have not opened for longest goes first.
Renskrift does not connect to the internet on its own
Diagrams and formulas are drawn on your device, offline.
The one exception is yours to turn on: Load Remote Images
A note can show a picture that lives on a website. Renskrift does not load such pictures unless you turn on View ▸ Load Remote Images (on iPhone and iPad, in the Writing menu). It is off until you do. While it is on, the toolbar shows that it is, and you can turn it off there or in the same menu at any time.
When it is on, Renskrift downloads a note's web pictures directly from the websites that host them, over secure (https) connections only. As with any web page, that website can see your device's internet address, the time, and which picture was asked for, so it may be able to tell that the note was opened. Renskrift sends it no cookies, sign-in details or anything from your notes. Downloaded pictures are kept in Renskrift's cache on your device, which the system may clear. Renskrift has no control over what those websites do with the requests they receive.
Crash reports
Crash reports reach the developer only if you have chosen to share them with app developers in your device's settings. They are collected and anonymised by Apple, under Apple's privacy policy.
Questions
Write to renskrift@carlill.com.