Remove "Protected:" From Titles of Password Protected Posts and Pages. Source: https://developer.wordpress.org/reference/hooks/protected_title_format/

remove-protected-from-titles.php

<?php

//* Remove "Protected:" from titles of password protected posts and pages
function customprefix_remove_protected_from_titles($title) {
    return '%s';
}
add_filter('protected_title_format', 'customprefix_remove_protected_from_titles');