Hide Widget Titles That Start With "!"

Lets you keep a descriptive widget title in the admin but hide it on the site, just by starting the title with an exclamation mark.

Code

add_filter( 'widget_title', function ( $title ) {
    return ( is_string( $title ) && 0 === strpos( $title, '!' ) ) ? '' : $title;
} );

Example: a widget titled "!Newsletter form" shows no heading on the site.

Install WPIDE on your WordPress site (it is free) and open Code Snippets.
Use snippet above adds it from the library in one click. Or copy the code into a new snippet with the same code type.
It is added inactive: review the code and settings, then switch it on.