Admin set all products to a global attribute

add_action( 'admin_notices', function() {

	// Attribute To Attach
	$attribute_id = wc_attribute_taxonomy_id_by_name( 'attribute-slug' );
	if( ! $attribute_id ) {
		echo 'Error: Attribute not found.';
		return;
	}

	// Get Products
	$args = [ 'limit' => -1 ];
	$products = wc_get_products( $args );
	foreach( $products as $product ) {

		$coo = $product->get_attribute( 'attribute-slug' );
		if( $coo ) {
			continue;
		}

		$attribute = new WC_Product_Attribute();
		$attribute->set_id( $attribute_id );
		$attribute->set_name( 'pa_attribute-slug' );
		$attribute->set_options( [ 'Attribute Value' ] );
		$attribute->set_visible( 1 );
		$attribute->set_variation( 0 );
		$attribute->set_position( 0 );

		$attributes = $product->get_attributes();
		$attributes['Attribute Title'] = $attribute;
		$product->set_attributes( $attributes );
		$product->save();
		printf( '+%d ', $product->get_id() );

	}

} );