A class to index all post types on all blogs in a WordPress multi-site setup. This is helpful since function "get_post_types" does not respect multisite functions like "switch_to_blog" etc.

multisite-post-type-index.class.php

<?php

/**
 * Class Multisite_Post_Type_Index
 *
 * This class creates and index of all the different post types on each blog in a multisite. As of now WordPress does not have a feature to list post types by blog, and hence this hacky but working solution.*
 */
class Multisite_Post_Type_Index {

	/**
	 * The option name we would like to store post types with.
	 *
	 * @var string
	 */
	private $option_name = 'registered_post_types';

	/**
	 * Singleton instance.
	 *
	 * @var null
	 */
	private static $instance = null;

	/**
	 * Instantiate class.
	 *
	 * @return Multisite_Post_Type_Index|null
	 */
	public static function get_instance() {
		if ( self::$instance == null ) {
			self::$instance = new Multisite_Post_Type_Index;
		}
		return self::$instance;
	}

	/**
	 * Multisite_Post_Type_Index constructor.
	 */
	private function __construct() {
		if ( ! is_multisite() ) return;
		add_action( 'registered_post_type', [$this, 'index_multisite_post_types'], 10, 2 );
	}

	/**
	 * Index the post type that gets registered.
	 *
	 * @param $post_type_name
	 * @param $post_type_object
	 */
	public function index_multisite_post_types( $post_type_name, $post_type_object ) {
		$registered_post_types = $this->get_post_types();
		$blog_id_key = $this->get_current_blog_id_key();
		if ( ! $this->should_index_post_type($post_type_object) ) return;
		if ( ! array_key_exists($blog_id_key, $registered_post_types) ) $registered_post_types[$blog_id_key] = [];
		if ( ! array_key_exists($post_type_name, $registered_post_types[$blog_id_key]) ) {
			$registered_post_types[$blog_id_key][$post_type_name] = $this->format_post_type_object($post_type_object);
			$this->set_post_types($registered_post_types);
		}
	}

	/**
	 * Format the post type object before storing it.
	 *
	 * @param $post_type_object
	 *
	 * @return mixed
	 */
	private function format_post_type_object($post_type_object) {
		return apply_filters('multisite_post_type_index_formatting', $post_type_object);
	}

	/**
	 * Get all post types in the multisite.
	 *
	 * @param $prefix_post_type_with_blog_id
	 *
	 * @return mixed
	 */
	public function get_all_post_types_in_multisite($prefix_post_type_with_blog_id = false) {
		$registered_post_types = $this->get_post_types();
		$post_types = [];
		foreach ( $registered_post_types as $blog_id => $post_types ) {
			foreach ( $post_types as $post_type ) {
				$key = ( $prefix_post_type_with_blog_id ? $blog_id . '_' . $post_type : $post_type );
				$post_types[$key] = $post_type;
			}
		}
		retun array_unique($post_types);
	}

	/**
	 * Get post types by blog Id.
	 *
	 * @param $blog_id
	 *
	 * @return bool|mixed
	 */
	public function get_post_types_for_blog($blog_id) {
		$registered_post_types = $this->get_post_types();
		$blog_id_key = $this->get_blog_id_key($blog_id);
		if ( ! array_key_exists($blog_id_key, $registered_post_types) ) {
			$post_types = $registered_post_types[$blog_id_key];
			if ( is_array($post_types) ) {
				$post_types = array_map(function ($post_type) {
					return (object) $post_type;
				}, $post_types);
				return $post_types;
			}
		}
		return false;
	}

	/**
	 * Check if we want to index this post type.
	 *
	 * @param $post_type_object
	 *
	 * @return bool
	 */
	private function should_index_post_type($post_type_object) {
		if ( ! $post_type_object->public ) {
			return false;
		}
		return true;
	}

	/**
	 * Get main blog Id.
	 *
	 * @return mixed
	 */
	private function main_blog_id() {
		return get_network()->site_id;
	}

	/**
	 * Get registered post types.
	 *
	 * @return array
	 */
	private function get_post_types() {
		return (array) get_blog_option($this->main_blog_id(), $this->option_name, []);
	}

	/**
	 * Set registered post types.
	 *
	 * @param $post_types
	 *
	 * @return mixed
	 */
	private function set_post_types($post_types) {
		return update_blog_option($this->main_blog_id(), $this->option_name, $post_types);
	}

	/**
	 * Clear registered post types.
	 */
	public function clear_post_types($post_types) {
		$this->set_post_types([]);
	}

	/**
	 * Get blog key for storage.
	 *
	 * @param $blog_id
	 *
	 * @return string
	 */
	private function get_blog_id_key($blog_id) {
		return 'blog_' . $blog_id;
	}

	/**
	 * Get main blog key for storage.
	 *
	 * @return string
	 */
	private function get_current_blog_id_key() {
		return $this->get_blog_id_key(get_current_blog_id());
	}

}
Multisite_Post_Type_Index::get_instance();