nicksmit

AdminNotes allows administrators and editors to add formatted notes to posts and pages. These notes can serve as reminders, tasks, or recommendations for content editors. Additionally, when enabled, users are notified to read these admin notes before editing the content.
Features: 
– Add formatted notes using a rich text editor in both Classic and Gutenberg editors. 
– Display a notification message to users when editing a post or page if admin notes are present. 
– Option to enable or disable the notification on a per-post/page basis.

AdminNotes

Adds an admin notes block to post and page editing screens with formatted text support and user notifications.

wycks

Like a built in notepad, leave notes for clients or yourself in the WordPress admin. Attach notes to above or below the editor of a post (or page or custom post type) and also has a default “Notes” section available from the admin menu which you can hide or show.
The style of the notes section has been updated with the changes in WordPress 3.8+. Please note that the UI will not look great on WordPress version lower then 3.8 and there are no plans to fix this.
Features :
<ul>
<li>Uses WordPress’s built in custom post types</li>
<li>Functions with default WYSIWYG editor</li>
<li>Uses WordPress’s meta fields if set to display on post edit screens</li>
<li>Shows “Notes” on one page in tab format (can be excluded)</li>
<li>Will not show in front end or search</li>
<li>Requires default post privileges</li>
</ul>
Notes :
<ul>
<li>It is currently only possible to set one note per post and one location (above or below editor)</li>
<li>Works with password protection in default section</li>
</ul>
Please report issues to: <a href="https://github.com/wycks/Simple-Admin-Notes" rel="nofollow ugc">https://github.com/wycks/Simple-Admin-Notes</a>

Simple Admin Notes

Adds a simple "Notes" section to the admin menu or posts

AdminNotes vs Simple Admin Notes