How to delete Locked Notes when you've forgotten your custom password
After upgrading to a new iPhone, I discovered I'd forgotten the custom password I'd created to use with Notes. Thankfully, using the old phone I managed to open a couple by Face Recognition and transferred them to the new phone but I was still stuck with a small number of locked Notes on the new phone I couldn't open.
Research revealed Apple are unable to assist in such circumstances and 'on-line' comments indicated there was no way to delete locked Notes on the new phone without the original password. I did try deleting Locked Notes individually but nothing worked and I thought I'd be stuck with them for ever.
Then I had an idea which has worked for me. First, as advice suggests I created a new password for Notes and tested it to make sure it worked. Then within Notes I created a new Folder and named it Locked Notes. I then selected each of the locked notes in turn and moved each one into the newly created folder from the menu of options.
I then selected the option to delete the 'Locked Notes' Folder and to my delight it accepted the command and deleted the folder and its entire contents. This method successfully removed all previously inaccessible locked notes from my device and across all associated devices. I accept I've lost the contents of each locked note for good but at least I've been able to delete them from the device. Hope this helps someone.