This is not on iPhone but on MacBook, but I've also lost several important notes just because I accidentally pressed Cmd + A and, then Backspace key while using, and closed the notes app.
My solution was to change the shortcut of 'Select All' from Cmd + A to Ctrl + A only for the notes app, and it has been satisfying so far.
Howto: System Preferences > Keyboard > Shortcuts > App Shortcuts (left pane) > Click + button and add 'notes' > Register new shortcut for 'Select All' as Ctrl + A.