Bill Erickson

Lets users know this site has been migrated elsewhere (useful so users don’t keep editing content on development site).
This plugin provides you with two editors for the notice displayed on the frontend and backend. The default message is “This site has been migrated,” but I like to modify it to include a link to the actual site.
<a href="https://github.com/billerickson/migration-notice/issues" rel="nofollow ugc">Support Forum</a>

Migration Notice

Lets users know this site has been migrated elsewhere.

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.

Migration Notice vs WP BrowserUpdate