Example REST Routes

https://example.com/wp-json/wpse/v1/latest-post-per-site
https://example.com/wp-json/wpse/v1/latest-post-per-site?number=10
https://example.com/wp-json/wpse/v1/latest-post-per-site?debug

Example Plugin

<?php
/**
 * Plugin Name: WPSE-307040: Rest API - Latest Post Per Site
 * Description: Rest API - latest post per site (multisite).
 * Plugin URI:  https://wordpress.stackexchange.com/a/307051/26350
 * Author:      birgire
 * Version:     1.0.0
 */

namespace WPSE\\RestAPI\\LatestPostPerSite;

/**
 * Register Rest Route
 */

add_action( \'rest_api_init\', function() {
    register_rest_route(
        \'wpse/v1\',
        \'/latest-post-per-site/\',
        [
           \'methods\'       => \'GET\',
           \'callback\'      => __NAMESPACE__.\'\\rest_results\'
        ]
    );
} );


/**
 * Rest Results
 */
function rest_results( $request ) {
    $parameters = $request->get_query_params();

    // Default 10 items
    $number = isset( $parameters[\'number\'] ) ? (int) $parameters[\'number\'] : 10;

    // Max 30 items
    if( $number > 30 ) {
        $number = 30;
    }

    $items = [];
    $i = 0;

    $blogs = get_sites( [
        \'orderby\' => \'last_updated\',
        \'order\'   => \'DESC\',
        \'number\'  => (int) $number         
    ] );

    foreach ( $blogs as $blog ) {
        switch_to_blog( $blog->blog_id );

        // Site info
        $details =  get_blog_details( $blog->blog_id );

        $items[$i][\'sitename\'] = esc_html( $details->blogname );
        $items[$i][\'siteid\']   = (int) $blog->blog_id;
        $items[$i][\'homeurl\']  = esc_url( $details->home );

        // Latest post
        $args = [
            \'orderby\'                 => \'post_date\',
            \'order\'                   => \'DESC\',
            \'posts_per_page\'          => 1,
            \'post_type\'               => \'post\',
            \'post_status\'             => \'publish\',
            \'ignore_sticky_posts\'     => true,
            \'no_found_rows\'           => true,
            \'update_post_term_cache\'  => false,
            \'update_post_meta_cache\'  => false,
        ];

        $query = new \\WP_Query( $args );

        if( $query->have_posts() ) {
            while( $query->have_posts() ) {
                $query->the_post();
                // Title
                $items[$i][\'title\']   = esc_html( get_the_title( get_the_ID() ) );
                // Date
                $items[$i][\'date\']    = esc_html( get_the_date( \'Y-m-d H:i:s\', get_the_ID() ) );
                // Unix time
                $items[$i][\'utime\']   = esc_html( get_the_date( \'U\', get_the_ID() ) );
                // Excerpt
                $items[$i][\'excerpt\'] = esc_html( wp_trim_words( strip_shortcodes( get_the_content() ), 50 ) );
                 // Permalink
                 $items[$i][\'url\']    = esc_url( get_permalink( get_the_ID() ) );
            }
            wp_reset_postdata();
       }

       $i++;
       restore_current_blog();

       // Sort by utime.
       $items = wp_list_sort( $items, \'utime\', \'DESC\' );

       $data = [
           \'success\' => true,
           \'count\'   => count( $items ),
           \'items\'   => $items,
       ];

       // Debug for super admins.
       if( isset(  $parameters[\'debug\'] ) && current_user_can( \'manage_networks\' ) ) {
           $data[\'qrs\'] = get_num_queries();
           $data[\'sec\'] = timer_stop(0);
       }

       return $data;
   }

希望有帮助！