jbalde

UserAgent Theme Switcher is a simple plugin which lets you change the theme to be displayed according to the detected browser.
Checks from a predefined list useragent so that the user can at will change the theme shown for that browser. 
A user can configure a special theme for example for mobile phones that is lighter and this adapted to these devices
Ideal for displaying themes ready to opera mobile, internet explorer mobile, iphone, android, etc.
Now the cache is not supported but will work so that if it is supported in the future

UserAgent Theme Switcher

UserAgent Theme Switcher is a simple plugin which lets you change the theme to be displayed according to the detected browser

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.

UserAgent Theme Switcher vs WP BrowserUpdate