Adding "Undo" to the Notes app??

Any deleted notes, will be available for 30 days in the "Recently deleted" folder of the group the note belonged to if the option is supported.

Tap on the Folders option at the top right corner, and then look for a "Recently Deleted" group there under the account the note was stored in. You can then recover the deleted note from there by tapping on the note to open it, then tapping in the note again to bring up the Recovery option.

click here ➜ Use Notes on your iPhone, iPad, and iPod touch - Recover a deleted Note - Apple Support

You can offer feedback to Apple if you want at Feedback - Notes - Apple

Hello sprucepie!

Firstly, please make sure your iPhone 6s Plus is fully updated to iOS 15.7.7.

I am not sure if the Undo feature was added in iOS 16, or not. Please check if you can do what I will instruct you to do in the following steps.

1. Go to: Settings > Accessibility > Touch. Now, turn on “Shake to Undo.”
2. Go to Notes.
3. Open a New Note, and write a phrase. Now, using your thumb on one side, and the first two fingers of the same hand on the other side, try shaking your iPhone very quickly, as if there was an axis fixed on the center of the device. You should get a pop up asking if you want to undo, redo, etc.

You can tell Apple about your concerns with Noted here: Feedback - Notes - Apple

If you have further questions, please reach out. I realize the above may be confusing, or not followable at all!

I hope this is helpful. Good Luck!