WordPress - Check network site for post before 404'ing

functions.php

<?php

/**
 * This filter will attempt to fetch a post from the main site if a post cannot be found
 * on a child site.
 **/
add_filter( 'the_posts', function( $posts, $wp_query ) {
        $network_id = get_network()->site_id;

        if ( ( ( count( $posts ) === 0 && is_single() ) || is_archive() || is_home() ) && get_current_blog_id() !== $network_id ) {
                switch_to_blog( $network_id );
                $network_query = new WP_Query();
                $network_query->query( $wp_query->query_vars );
                $posts = $network_query->posts;
                restore_current_blog();

                foreach ( $posts as $key => $post ) {
                        $localised = get_posts( [
                                'name' => $post->post_name,
                                'posts_per_page' => 1,
                        ] );
                        if ( count( $localised === 1 ) && is_a( $localised[0], 'WP_Post' ) ) {
                                $posts[ $key ] = $localised[0];
                        }
                }
        }

        return $posts;
}, 10, 2 );