woocommerce_email_order_meta.php

<?php

add_action( 'woocommerce_email_order_meta', 'ts_email', 10, 4 );
function ts_email( $order, $sent_to_admin, $plain_text, $email ) {
	$items = $order->get_items(); 

    foreach ($items as $key => $item) {
        $product_name = $item['name'];
        $product_id = $item['product_id'];
        $terms = get_the_terms( $product_id, 'product_cat' );

		$args = array(
			'post_type'      => 'product',
			'posts_per_page' => 10,
			'product_cat'    => $terms[0]->slug
		);
	
		$loop = new WP_Query( $args );
		echo 'Here are few more products based on your recent purchase';
	
		while ( $loop->have_posts() ) : $loop->the_post();
			global $product;
			echo '<br><a href="'.get_permalink().'">' . get_the_title().'</a>';
		endwhile;
	
		wp_reset_query();
	}
}