Woocommerce email order items filter by categories

functions.php

<?php
function modify_wc_email_order_items_args( $args ) {
    if( isset( $args['order'] ) && $args['order'] ){
        $items = ( isset( $args['items'] ) && $args['items'] ) ? $args['items'] : array();
        if( $items ) :
            $cat_wisw_pros = array();
            foreach ( $items as $item_id => $item ){
                $product_id = $item['product_id'];
                $cat_ids = wp_get_post_terms( $product_id, 'product_cat', array( 'fields' => 'ids' ) );
                foreach ( $cat_ids as $id ) {
                    $cat_wisw_pros[$id][$item_id] = $item;
                }
            }
            ksort( $cat_wisw_pros ); // Cat ID wise sort
            $grouped_order_items = array();
            foreach ( $cat_wisw_pros as $cat_id => $order_items ) {
                foreach ( $order_items as $item_id => $item ) {
                    if( !array_key_exists( $item_id, $grouped_order_items ) )
                        $grouped_order_items[$item_id] = $item;
                }
            }
            $args['items'] = $grouped_order_items;
        endif;
    }
    return $args;
}
add_filter( 'woocommerce_email_order_items_args', 'modify_wc_email_order_items_args', 99 );