Sed Lex

<p>Add an update box in posts.</p>
<p>This box can contain a message, for instance in order to point out that the post have been modified of to stress that the post in no longer up to date</p>
<p>The message can be configured direcly when editing a post. There is a box ‘Update message’ added on the left.</p>
<p>In addition, you may use a shortcode [maj update=’dd/mm/yy’ expire=’dd/mm/yy’]xxx[/maj]</p>
<p>Plugin developped from the orginal plugin WP Update Message.</p>
<p>This plugin is under GPL licence.</p>
<h4>Multisite – WordPress MU</h4>
<p>This plugin works with multisite installation of WordPress.</p>
<h4>Localization</h4>
<ul>
<li>German (Germany) translation provided by klotzi703611, RobertBergholz</li>
<li>Greek (Greece) translation provided by skyrodos</li>
<li>English (United States), default language</li>
<li>Spanish (Spain) translation provided by BrenardCrespo, notaspampeanas</li>
<li>French (France) translation provided by SedLex</li>
<li>Italian (Italy) translation provided by DarioDaniele, Androidous</li>
<li>Dutch (Netherlands) translation provided by MichaelWijnands</li>
<li>Polish (Poland) translation provided by Kajaczek</li>
<li>Portuguese (Portugal) translation provided by PedroNave</li>
<li>Russian (Russia) translation provided by MegaJeme, RobertBergholz</li>
<li>Turkish (Turkey) translation provided by kbasali</li>
<li>Chinese (People’s Republic of China) translation provided by wensonjo</li>
</ul>
<h4>Features of the framework</h4>
<p>This plugin uses the SL framework. This framework eases the creation of new plugins by providing tools and frames (see dev-toolbox plugin for more info).</p>
<p>You may easily translate the text of the plugin and submit it to the developer, send a feedback, or choose the location of the plugin in the admin panel.</p>
<p>Have fun !</p>


Update Message

Jonathan Bardo

<p><strong>Note: This plugin only works with WordPress 4.2 or higher</strong></p>
<p>This plugin will enable the site administrator to easily create an admin message present on all pages. The admin has the option to let the user dismiss the message for the duration of his session.</p>


Admin User Message

This does just what I want. Thanks for making it. :)
It's definitely better than https://wordpress.org/plugins/admin-screen-messages
You might want to also tag it as:
https://wordpress.org/plugins/tags/admin-message



Perfect

Add message to users of your site. Choose wheter they can dismiss it or not.

Update Message vs Admin User Message