backorder_override.php

<?php 

function wpcb_should_override_backorder( $product ) {

    //Set the product categories in the array
    $categories = array('cat-a', 'cat-b');
    
    return has_term( $categories, 'product_cat' ) && $product->get_attribute('pa_attr-a');
}

function __enable_backorders( $value, $product ) {

    // Allow Backorder For All Products In Specific Categories WITH Specific Attribute
    if( wpcb_should_override_backorder($product) ){
    
        // Enables Manage Stock - Use "yes" or "notify", which is the equivalent of "Allow, but notify customer"
        return $product->get_manage_stock('edit') ? $value : 'notify';

    }
}

function woocommerce_product_get_stock_status( $stock_status, $product ) {

    if(wpcb_should_override_backorder($product)){
        return 'onbackorder';
    }
}

add_filter( 'woocommerce_product_get_backorders', '__enable_backorders', 0, 2);
add_filter( 'woocommerce_product_get_stock_status', 'woocommerce_product_get_stock_status', 10, 2 );