Dana Ross

This plugin displays a message at the top of the screen for visitors running outdated versions of the most popular browsers:
<ul>
<li>Internet Explorer 6 SP2 or less</li>
<li>FireFox 2 or less</li>
<li>Safari 2 or less</li>
</ul>
It is designed to be unobtrusive. The message disappears after a few seconds, and it only displays the first time the visitor comes to your WordPress blog/site.
The warning reads: It appears that you are using an outdated version of your browser. Please upgrade your browser for better sercurity and an improved experience.
The message also includes a link where the newest version of the browser can be downloaded.

Dave’s Outdated Browser Warning

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.
WP BrowserUpdate is based on the browser-update.org detection logic. The plugin now runs independently from browser-update.org at display time: the visitor-facing notification is served from the plugin’s own bundled runtime files, so sites do not need to load scripts, styles, or default notification links from browser-update.org. browser-update.org remains the credited upstream source for the bundled detection logic, and future runtime refreshes are tracked with source URLs and hashes in the plugin assets.
Visit <a href="https://browserupdate.org/" rel="nofollow ugc">browserupdate.org</a> for more details about the upstream project.
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>How it works</h3>
WP BrowserUpdate bundles browser-update.org detection logic for WordPress. After activation, the plugin loads the local notification runtime from the plugin directory and passes your configured browser-version thresholds to those scripts. The notification is shown only when the bundled detection logic matches a browser to your settings.
The settings page is available under Settings > WP BrowserUpdate. You can define browser versions for every browser key supported by the bundled browser-update.org runtime, choose where the message appears and which element should contain it, enable testing mode, decide whether mobile or unsupported browsers should be notified, customize links, language and message text, and add trusted custom CSS for the notification.
Browser version fields accept major versions such as <code>115</code> and positive dotted versions such as <code>137.0.3912.63</code>. Dotted versions are passed exactly to the bundled runtime instead of being reduced to their major version; exact comparison depends on the bundled browser-update.org logic and the browser key used by that runtime. A value of <code>0</code> uses the default bundled outdated-browser detection. Negative whole numbers are passed to the bundled runtime as relative offsets from the current bundled upstream version.
Microsoft Edge and Microsoft Internet Explorer have separate settings now.
This local runtime design avoids frontend blocking of external script URLs on sites with strict Content Security Policies or tracker blocking. WP BrowserUpdate ships only the local runtime and CSP adapter files needed by the plugin.
<h3>Important Notice</h3>
Breaking Changes in Version 6.0 
– Requires WordPress 6.0 or newer. 
– Introduces a new structured settings model to support the browser-update.org customization surface used by WP BrowserUpdate. 
– Moves the visitor-facing browser-update.org runtime into the plugin package so strict Content Security Policies and tracker blockers no longer need to allow scripts from <code>browser-update.org</code>. 
– Existing WP BrowserUpdate settings from version 5.x are migrated automatically.
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.

Dave’s Outdated Browser Warning vs WP BrowserUpdate