MacSteini

Many users still browse with outdated browsers, often unaware of the risks. Upgrading ensures better security and reliability. This plugin displays a subtle notification prompting visitors to update their browser. Activate the plugin, and it works seamlessly.
Visit <a href="https://browserupdate.org/" rel="nofollow ugc">browserupdate.org</a> for more details.
Want to help translate this plugin? Visit the <a href="https://translate.wordpress.org/projects/wp-plugins/wp-browser-update" rel="nofollow ugc">WordPress Translation Project</a>.
<h3>Important Notice</h3>
Breaking Changes in Version 5.0 
– Requires PHP 7.4 or newer. 
– Ensure your hosting is updated to PHP 7.4 before upgrading to version 5.0 or newer. 
– Servers running older PHP versions are no longer supported. 
 – If your server is running an earlier PHP version, please download <a href="https://downloads.wordpress.org/plugin/wp-browser-update.4.8.1.zip" title="Download WP BrowserUpdate from WordPress.org" rel="nofollow ugc">version 4.8.1</a>.

WP BrowserUpdate

Used for a client who had this exact request. Worked well.

Works as expected

This plugin notifies website visitors to update their outdated browser in a non-intrusive way.

Mario Peshev

DevriX

DX Out of Date allows you to display a notification box on your posts 
when a given amount of time has passed. Quite handy when writing tutorials 
or any content that might get outdated due to external factors – now you 
could automatically notify your readers after a given period.
You can configure the expiration time of your posts in the admin panel. Valid time units are days, months and years.
Currently the plugin options are:
<ul>
<li>Period – “amount” of “duration” units (1 to 40)</li>
<li>Duration frame (days, months, years)</li>
<li>Message – the message format for the notification box</li>
<li>Enable by default – a checkbox for enabling the message on all posts by default</li>
<li>Enable post status in Dashboard – a checkbox for adding an extra column in the Dashboard for the outdated status of posts</li>
<li>Post types to affect – allows you to choose which post types for display the message for</li>
<li>Position of the message – choose whether the message should be displayed at the top or at the bottom of a post’s content</li>
<li>Background color – Choose a background color for the message</li>
<li>Text color – Choose a text color for the message</li>
<li>Custom CSS – Add custom CSS rules to the message box</li>
</ul>

DX Out of Date

Display a notification box on the single view of your outdated posts, given a set amount of time (configurable).

WP BrowserUpdate vs DX Out of Date