Disable Delivery Date Based On Chosen Shipping Method

/**
 * Modify dates based on shipping method.
 *
 * @param array  $available_dates
 * @param string $format
 * @param bool   $ignore_slots
 *
 * @return array
 */
function iconic_modify_delivery_dates( $available_dates, $format, $ignore_slots ) {
	global $jckwds;

	$chosen_method = $jckwds->get_chosen_shipping_method();

	if ( 'flat_rate:3' === $chosen_method ) {
		// Remove these dates if chosen method is flat_rate:5.
		$remove = array( '20210102', '20210103' );

		// If the format is not array, then we need to cast the
		// "remove" dates to the  required format.
		if ( 'array' !== $format ) {
			foreach ( $remove as $key => $ymd ) {
				$remove[ $key ] = date( $format, strtotime( $ymd ) );
			}
		}

		foreach ( $available_dates as $key => $available_date ) {
			$date = 'array' === $format ? $available_date['ymd'] : $available_date;

			if ( ! in_array( $date, $remove, true ) ) {
				continue;
			}

			unset( $available_dates[ $key ] );
		}
	}

	// `array_values()` resets the array keys so the default date is index "0".
	return array_values( $available_dates );
}

add_filter( 'iconic_wds_available_dates', 'iconic_modify_delivery_dates', 10, 3 );