Cloud syncing of data is one of those things we've come to rely on. But we also trust cloud services to protect our data. And that extends to deletions. ElcomSoft has proven that it is possible to retrieve deleted notes after the 30-day window that Apple has set for getting notes back.
Deleted notes are stored in the Recently Deleted folder in the iOS and macOS Notes application. After 30 days, the notes disappear. But it seems Apple isn't actually clearing the data from their stores.
Elcomsoft has been able to find deleted notes going all the way back to 2015,. They've told Apple about this, just as they did with a couple of similar issues they detected previously, which Apple has previously patched.
In short, if you're using Notes for iOS or macOS, you can't be 100% confident data you delete is actually gone until Apple resolves the issue.