A WooCommerce code snippet that forces the 'state' field to always display on checkout pages.

woocommerce-force-display-state.php

<?php
/**
 * Modifies the billing address fields to make the state field required.
 *
 * @param array $address_fields The array of address fields used in WooCommerce checkout.
 * @return array The modified array of address fields with 'state' field set as required.
 */
function force_state_field_default_required( $address_fields ) {
    // Set the 'required' flag for the 'billing_state' field to true.
    $address_fields['billing_state']['required'] = true;

    // Return the modified address fields array.
    return $address_fields;
}

// Add a filter to WooCommerce to apply the above function to the billing fields.
add_filter( 'woocommerce_billing_fields', 'force_state_field_default_required');

/**
 * Modifies the locale settings to make the state field required for all countries.
 *
 * @param array $locale The array of country locales provided by WooCommerce.
 * @return array The modified array of locales with 'state' field set as required for all countries.
 */
function force_state_field_locale_required( $locale ) {
    // Access the global locales variable.
    global $locales;

    // Get all country codes from WooCommerce.
    $country_codes = array_keys( WC()->countries->get_countries() );

    // Initialize the locales array.
    $locales = array();

    // Loop through each country code.
    foreach ( $country_codes as $country_code ) {
        // Set the 'state' field to be required for the current country.
        $locales[$country_code] = array(
            'state' => array(
                'required' => true
            ),
        );
    }

    // Return the modified locales array.
    return $locales;
}

// Add a filter to WooCommerce to apply the above function to the country locale settings.
add_filter( 'woocommerce_get_country_locale', 'force_state_field_locale_required');