add_filter( 'woocommerce_get_price_html', 'wpcb_hide_regular_price_for_sale_products', 10, 2 );
if ( ! function_exists( 'wpcb_hide_regular_price_for_sale_products' ) ) {
    /**
     * wpcb_hide_regular_price_for_sale_products.
     *
     * @see  https://wpcodebook.com/snippets/hide-regular-price-for-products-on-sale-in-woocommerce/
     */
    function wpcb_hide_regular_price_for_sale_products( $price, $product ) {
        if ( $product->is_on_sale() ) {
            if ( $product->is_type( 'variable' ) ) {
                // Variable products
                $prices = $product->get_variation_prices( true );
                if ( ! empty( $prices['price'] ) ) {
                    $min_price     = current( $prices['price'] );
                    $max_price     = end( $prices['price'] );
                    $min_reg_price = current( $prices['regular_price'] );
                    $max_reg_price = end( $prices['regular_price'] );
                    if ( $min_price === $max_price && $min_reg_price === $max_reg_price ) {
                        return wc_price( $min_price ) . $product->get_price_suffix();
                    }
                }
            } elseif ( ! $product->is_type( 'grouped' ) ) {
                // Simple etc. products (i.e. all except grouped)
                return wc_price( wc_get_price_to_display( $product ) ) . $product->get_price_suffix();
            }
        }
        // No changes
        return $price;
    }
}