Replace Upsells with products from a specific product category in Woocommerce 3

To replace woocommerce upsells by products from a specific product category use the following code (where you will define your product category(ies) slug(s)):

add_filter( 'woocommerce_product_get_upsell_ids', 'custom_upsell_ids', 20, 2 );
function custom_upsell_ids( $upsell_ids, $product ){
    // HERE define your product categories slugs in the array
    $product_categories = array( 't-shirts' ); // <=====  <=====  <=====  <=====  <=====

    // Return the custom query
    return wc_get_products( array(
        'status'    => 'publish', // published products
        'limit'     => 4, // 4 products
        'category'  => $product_categories, // Product categories
        'orderby'   => 'rand', // Random order
        'exclude'   => array( $product->get_id() ), // Exclude current product
        'return'    => 'ids', // Query returns only IDs
    ) );
}

Code goes in function.php file of the active child theme (or active theme). Tested and works.

The query is set to display 4 products from a specific product category in a random order (excluding the current product).

Official documentation: The wc_get_products() and WC_Product_Query