Magento:: Show ‘Out of stock’ simple products in the configurable product?

To achieve this feature you need to override magento core function follow the steps:
1. First, we need to create a module enabler file. Create a file “app/etc/modules/Sushant_Catalog.xml”

<?xml version="1.0"?>
<config>
    <modules>
        <Sushant_Catalog>
            <active>true</active>
            <codePool>local</codePool>
        </Sushant_Catalog>
    </modules>
</config>

2. Next, we need to create a module configuration file. Create “app/code/local/Sushant/Catalog/etc/config.xml” and paste the following contents in that file.

<?xml version="1.0"?>
<config>
    <modules>
        <Sushant_Catalog>
            <version>1.0</version>
        </Sushant_Catalog>
    </modules>
   
    <global>
        <blocks>
            <catalog>
                <rewrite>
                    <product_view_type_configurable>Sushant_Catalog_Block_Product_View_Type_Configurable</product_view_type_configurable>
                </rewrite>
            </catalog>
        </blocks>
    </global>
</config>

3. We need to define a block class Sushant_Catalog_Block_Product_View_Type_Configurable. Let’s create a block file “app/code/local/Sushant/Catalog/Block/Product/View/Type/Configurable” with the following contents.

<?php

class Sushant_Catalog_Block_Product_View_Type_Configurable extends Mage_Catalog_Block_Product_View_Type_Configurable {
    
    public function getJsonConfig() {
        $attributes = array();
        $options    = array();
        $store      = $this->getCurrentStore();
        $taxHelper  = Mage::helper('tax');
        $currentProduct = $this->getProduct();

        $preconfiguredFlag = $currentProduct->hasPreconfiguredValues();
        if ($preconfiguredFlag) {
            $preconfiguredValues = $currentProduct->getPreconfiguredValues();
            $defaultValues       = array();
        }

        foreach ($this->getAllowProducts() as $product) {
            $productId  = $product->getId();

            foreach ($this->getAllowAttributes() as $attribute) {
                $productAttribute   = $attribute->getProductAttribute();
                $productAttributeId = $productAttribute->getId();
                $attributeValue     = $product->getData($productAttribute->getAttributeCode());
                if (!isset($options[$productAttributeId])) {
                    $options[$productAttributeId] = array();
                }

                if (!isset($options[$productAttributeId][$attributeValue])) {
                    $options[$productAttributeId][$attributeValue] = array();
                }
                $options[$productAttributeId][$attributeValue][] = $productId;
            }
        }

        $this->_resPrices = array(
            $this->_preparePrice($currentProduct->getFinalPrice())
        );

        foreach ($this->getAllowAttributes() as $attribute) {
            $productAttribute = $attribute->getProductAttribute();
            $attributeId = $productAttribute->getId();
            $info = array(
               'id'        => $productAttribute->getId(),
               'code'      => $productAttribute->getAttributeCode(),
               'label'     => $attribute->getLabel(),
               'options'   => array()
            );

            $optionPrices = array();
            $prices = $attribute->getPrices();
            if (is_array($prices)) {
                foreach ($prices as $value) {
                    if(!$this->_validateAttributeValue($attributeId, $value, $options)) {
                        // continue;
                        $inStock = false;
                    }
                    $currentProduct->setConfigurablePrice(
                        $this->_preparePrice($value['pricing_value'], $value['is_percent'])
                    );
                    $currentProduct->setParentId(true);
                    Mage::dispatchEvent(
                        'catalog_product_type_configurable_price',
                        array('product' => $currentProduct)
                    );
                    $configurablePrice = $currentProduct->getConfigurablePrice();

                    if (isset($options[$attributeId][$value['value_index']])) {
                        $productsIndex = $options[$attributeId][$value['value_index']];
                    } else {
                        $productsIndex = array();
                    }

                    $info['options'][] = array(
                        'id'        => $value['value_index'],
                        'label'     => $value['label'],
                        'price'     => $configurablePrice,
                        'oldPrice'  => $this->_prepareOldPrice($value['pricing_value'], $value['is_percent']),
                        'products'  => $productsIndex,
                        'stock'     => $inStock 
                    );
                    $optionPrices[] = $configurablePrice;
                }
            }
            /**
             * Prepare formated values for options choose
             */
            foreach ($optionPrices as $optionPrice) {
                foreach ($optionPrices as $additional) {
                    $this->_preparePrice(abs($additional-$optionPrice));
                }
            }
            if($this->_validateAttributeInfo($info)) {
               $attributes[$attributeId] = $info;
            }

            // Add attribute default value (if set)
            if ($preconfiguredFlag) {
                $configValue = $preconfiguredValues->getData('super_attribute/' . $attributeId);
                if ($configValue) {
                    $defaultValues[$attributeId] = $configValue;
                }
            }
        }

        $taxCalculation = Mage::getSingleton('tax/calculation');
        if (!$taxCalculation->getCustomer() && Mage::registry('current_customer')) {
            $taxCalculation->setCustomer(Mage::registry('current_customer'));
        }

        $_request = $taxCalculation->getDefaultRateRequest();
        $_request->setProductClassId($currentProduct->getTaxClassId());
        $defaultTax = $taxCalculation->getRate($_request);

        $_request = $taxCalculation->getRateRequest();
        $_request->setProductClassId($currentProduct->getTaxClassId());
        $currentTax = $taxCalculation->getRate($_request);

        $taxConfig = array(
            'includeTax'        => $taxHelper->priceIncludesTax(),
            'showIncludeTax'    => $taxHelper->displayPriceIncludingTax(),
            'showBothPrices'    => $taxHelper->displayBothPrices(),
            'defaultTax'        => $defaultTax,
            'currentTax'        => $currentTax,
            'inclTaxTitle'      => Mage::helper('catalog')->__('Incl. Tax')
        );

        $config = array(
            'attributes'        => $attributes,
            'template'          => str_replace('%s', '#{price}', $store->getCurrentCurrency()->getOutputFormat()),
            'basePrice'         => $this->_registerJsPrice($this->_convertPrice($currentProduct->getFinalPrice())),
            'oldPrice'          => $this->_registerJsPrice($this->_convertPrice($currentProduct->getPrice())),
            'productId'         => $currentProduct->getId(),
            'chooseText'        => Mage::helper('catalog')->__('Choose an Option...'),
            'taxConfig'         => $taxConfig
        );

        if ($preconfiguredFlag && !empty($defaultValues)) {
            $config['defaultValues'] = $defaultValues;
        }

        $config = array_merge($config, $this->_getAdditionalConfig());

        return Mage::helper('core')->jsonEncode($config);
    }
}