Make wordpress synced blocks public in rest api

functions.php

<?php
// fetch block by name like this:
// https://…/wp-json/public/v1/reusable?slug=brunch-termine
add_action('rest_api_init', function () {
  register_rest_route('public/v1','/reusable',[
    'methods'  => 'GET',
    'args'     => ['slug'=>['required'=>true]],
    'permission_callback' => '__return_true',
    'callback' => function(WP_REST_Request $r){
      $slug = sanitize_title($r['slug']);
      $post = get_page_by_path($slug, OBJECT, 'wp_block');
      if(!$post || 'publish'!==$post->post_status){
        return new WP_Error('not_found','Block not found or not public',['status'=>404]);
      }
      return [
        'id'       => $post->ID,
        'slug'     => $post->post_name,
        'title'    => get_the_title($post),
        'rendered' => apply_filters('the_content', $post->post_content),
      ];
    }
  ]);
});