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add_filter('woocommerce_checkout_fields', function($fields) {
    foreach ($fields['billing'] as $key => $field) {
        $fields['billing'][$key]['custom_attributes']['readonly'] = 'readonly';
    }
    return $fields;
});

add_filter('woocommerce_billing_fields', function($fields) {
    foreach ($fields as $key => $field) {
        $fields[$key]['custom_attributes']['readonly'] = 'readonly';
    }
    return $fields;
});