I really have no idea if this plugin is even useful to anyone. There are ways to get the folder id from the debug console, but I didn't want to search for it.
I doubt that it makes any sense for a regular user to install it. It only reads data thus it is inherently safe to use.
Get Notebook ID
Copy/Show the ID of a notebook. For developers. A standard user won't need this.
Settings
Please note: All settings require a restart of Joplin.
Copy Notebook ID: Enable 'Copy Notebook ID' in the notebook context menu. It will copy the notebook ID to the clipboard.
Show Notebook ID: Enable 'Show Notebook ID' in the notebook context menu. It will open a dialog and display the notebook ID. The dialog also makes it possible to copy the ID to the clipboard.