The following version code will automatically remove products from the "Sale" category when they are not anymore on sale:
add_action( 'save_post_product', 'update_product_set_sale_cat' );
function update_product_set_sale_cat( $post_id ) {
if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
return $post_id;
}
if ( ! current_user_can( 'edit_product', $post_id ) ) {
return $post_id;
}
if( get_post_status( $post_id ) == 'publish' && isset($_POST['_sale_price']) ) {
$sale_price = $_POST['_sale_price'];
if( $sale_price >= 0 && ! has_term( 'Sale', 'product_cat', $post_id ) ){
wp_set_object_terms($post_id, 'sale', 'product_cat', true );
} elseif ( $sale_price == '' && has_term( 'Sale', 'product_cat', $post_id ) ) {
wp_remove_object_terms( $post_id, 'sale', 'product_cat' );
}
}
}
Code goes in function.php file of your active child theme (or active theme). Tested and works.
Related: Adding "Sale" product category to products that are on sale in Woocommerce