By default when a comment is deleted, WordPress does not delete the replies of that comment. With this function whenever a comment is deleted, its replies will be automatically deleted.
/**
* Delete comment replies when deleting comment.
*/
add_action( 'delete_comment', function( $comment_id ) {
$child = get_comments([
'fields' => 'ids',
'parent' => $comment_id,
]);
if ( $child ) {
foreach( $child as $child_id ) {
wp_delete_comment( $child_id, true );
}
}
});