Implements a standardized messaging system that allows admin notices to be passed across page redirects. usage: add_filter( 'wp_persistent_notices', function( $notices ) { $notices[] = array( 'type' => 'error', 'message' => "Houston, we have a problem!" ); return $notices; } );

WP_Persistent_Notices.php

<?php if( !defined( 'ABSPATH' ) ) { die(); } // Include in all php files, to prevent direct execution
/**
 * Class Name : WP Persistent Notices
 * Description : Implements a Standardized messaging system that allows admin messages to be passed across page redirects.
 * Class URI : http://gschoppe.com/wordpress/pass-wordpress-admin-notices-across-page-redirects/
 * Version : 1.0.0
 * Author : Greg Schoppe
 * Author URI : http://gschoppe.com
 * License: GPL-2.0+
 * License URI: http://www.gnu.org/licenses/gpl-2.0.txt
 **/
if( !class_exists( 'WP_Persistent_Notices' ) ) {
	class WP_Persistent_Notices {  // replace ClassName with the name of your class
		private static $_this;
		private $session_var = 'wp_persistent_notices';
		private $max_age;
		private $default_notice;
		private $notices;

		public static function Instance() {
			static $instance = null;
			if ( $instance === null ) {
				$instance = new self();
			}
			return $instance;
		}

		private function __construct() {
			$this->max_age = max( intval( ini_get( 'max_execution_time' ) ) * 2, 30 );
			$this->notices = array();
			$this->default_notice = array(
				'id'           => '',
				'type'         => 'info',
				'custom_class' => '',
				'message'      => '',
				'dismissible'  => false,
				'html'         => '',
				'location'     => 'default'
			);
			add_action( 'admin_init'                    , array( $this, 'admin_init'       ) );
			add_action( 'admin_notices'                 , array( $this, 'render_notices'   ) );
			add_action( 'wp_ajax_get_persistent_notices', array( $this, 'ajax_get_notices' ) );
			add_action( 'shutdown'                      , array( $this, 'save_notices'     ) );
		}
		public function admin_init() {
			// add a filter and that returns true to replace or prevent the session initialization functionality
			if( !apply_filters( 'wp_persistent_notices_replace_initialization', false ) ) {
				session_start();
			}
		}
		public function add_notice( $notice = array() ) {
			if( !is_array( $notice ) ) {
				$notice = array(
					'message' => $notice
				);
			}
			$notice = array_merge( $this->default_notice, $notice );
			$notice['created'] = time();
			$this->notices[] = $notice;
			return true;
		}

		public function render_notices( $location = 'default' ) {
			if( !$location ) {
				$location = 'default';
			}
			if( defined( 'DOING_AJAX' ) && DOING_AJAX ) {
				return;
			}
			$notices = $this->retrieve_notices( $location );
			$output_array = array();
			foreach ( $notices as $notice ) {
				if( !is_array( $notice ) ) {
					$notice = array(
						'message' => $notice
					);
				}
				if( $notice['html'] ) {
					$output_array[] = wp_kses_post( $notice['html'] );
				} elseif( $notice['message'] ) {
					$notice = array_merge( $this->default_notice, $notice );
					$id = '';
					if( $notice['id'] ) {
						$id = ' id="wp-persistent-notice-' . sanitize_title( $id ) . '"';
					}
					$type = '';
					if( $notice['type'] ) {
						$type = ' notice-' . sanitize_title( $notice['type'] );
					}
					$cclass = '';
					if( $notice['custom_class'] ) {
						$cclass = ' ' . sanitize_title( $notice['custom_class'] );
					}
					$dismissible = '';
					if( $notice['dismissible'] ) {
						$dismissible = ' is-dismissible';
					}
					$class = ' class="notice' . $type . $cclass . $dismissible . '"';
					$message = wp_kses_post( $notice['message'] );
					$output_array[] = sprintf( '<div%1$s%2$s><p>%3$s</p></div>', $id, $class, $message );
				}
			}
			echo implode( "\n", $output_array );
		}

		public function ajax_get_notices() {
			$location = 'default';
			if( isset( $_GET['location'] ) && $_GET['location'] ) {
				$location = $_GET['location'];
			}
			if( isset( $_POST['location'] ) && $_POST['location'] ) {
				$location = $_POST['location'];
			}
			$notices = $this->retrieve_notices( $location );
			echo json_encode( $notices );
			wp_die();
		}

		public function save_notices() {
			$notices = $this->retrieve_notices( 'all' );
			if ( isset( $notices ) ) {
				// add a filter that returns true to replace the notice saving functionality
				if( apply_filters( 'wp_persistent_notices_replace_save_notices', false, $notices ) !== true ) {
					$_SESSION[ $this->session_var ] = $notices;
				}
			}
		}

		private function retrieve_notices( $location = 'default' ) {
			// note: once a notice is retrieved, it's gone
			$notices = array();
			// add a filter that returns true to prevent retrieving messages on specific pages
			if( !apply_filters( 'wp_persistent_notices_prevent_retrieval', false ) ) {
				// add a filter that returns notices or empty array() to replace the session retrieval functionality
				$raw_notices = apply_filters( 'wp_persistent_notices_replace_retrieve_notices', false );
				if( $raw_notices === false ) {
					$raw_notices = array();
					if( isset( $_SESSION[ $this->session_var ] ) && is_array( $_SESSION[ $this->session_var ] ) ) {
						$session_notices = $_SESSION[ $this->session_var ];
						if( is_array( $session_notices ) ) {
							$raw_notices = $session_notices;
						}
					}
				}
				$raw_notices += $this->notices;
				$raw_notices = apply_filters( 'wp_persistent_notices', $raw_notices );
				// clear all places notices are stored
				unset( $_SESSION[ $this->session_var ] );
				$this->notices = array();
				remove_all_filters( 'wp_persistent_notices' );
				$notices = array();
				// filter for age and location
				foreach( $raw_notices as $notice ) {
					if( !isset( $notice['created'] ) || !$notice['created'] ) {
						$notice['created'] = time();
					} else {
						if( time() - $notice['created'] > $this->max_age ) {
							continue;
						}
					}
					if( $location == 'all' || $notice['location'] == $location ) {
						$notices[] = $notice;
					} else {
						$this->notices[] = $notice;
					}
				}
				// attach a filter that returns true to disable grouping notices
				if( !apply_filters( 'wp_persistent_notices_display_ungrouped', false ) ) {
					usort( $notices, array( $this, 'compare_notices' ) );
				}
			}
			return $notices;
		}

		private function compare_notices( $a, $b ) {
			// attach a filter that returns an integer { -1, 0, 1 } to replace comparison function
			$compare = apply_filters( 'wp_persistent_notices_replace_comparison', false, $a, $b );
			if( $compare !== false ) {
				return $compare;
			}
			// if one of them doesn't have a type
			if( !isset( $a['type'] ) ) {
				if( !isset( $b['type'] ) ) {
					return 0;
				}
				return -1;
			}
			// if types are equal
			if( $a['type'] == $b['type'] ) {
				return 0;
			}
			// give a weight to each type
			$test = array( $a['type'], $b['type'] );
			$results = array();
			foreach( $test as $val ) {
				$i = false;
				switch( $val ) {
					case 'error':
						$i = apply_filters( 'wp_persistent_notices_sort_error_weight', 0 );
						break;
					case 'warning':
						$i = apply_filters( 'wp_persistent_notices_sort_warning_weight', 5 );
						break;
					case 'success':
						$i = apply_filters( 'wp_persistent_notices_sort_success_weight', 10 );
						break;
					case 'info':
						$i = apply_filters( 'wp_persistent_notices_sort_info_weight', 15 );
						break;
					default:
						// use the val to determine weight
						$i = apply_filters( 'wp_persistent_notices_sort_custom_weight', 20, $val );
				}
				$results[] = $i;
			}
			return ( $results[0] - $results[1] );
		}
	}
	WP_Persistent_Notices::Instance();

	if( !function_exists( 'add_persistent_notice' ) ) {
		function add_persistent_notice( $notice ) {
			$cl = WP_Persistent_Notices::Instance();
			return $cl->add_notice( $notice );
		}
	}
	if( !function_exists( 'render_persistent_notices' ) ) {
		function render_persistent_notices( $location = 'default' ) {
			$cl = WP_Persistent_Notices::Instance();
			return $cl->render_notices( $location );
		}
	}
}

Its code is very simple, has many features and uses transients.

your code does not solve one of the core issues with WordPress messages, which is the need for transactional messages that can survive over a double redirect. Your code targets notices to classes of user or all users, but not to the specific user who initiated an action. In multi-user environments, this means that one user might cause an action, and another might receive the notification.

Beyond that, it is considered very rude to spam pages for one solution to advertise your alternative.

Excuse me.

You're right. My code is not for general purpose WordPress usage.
It is for system usage when some central thing goes wrong: e.g. "Google API is not reachable" in wp-cron, or "All editors, please prepare for release!"