The message is generated by WordPress, and its generated because “WordPress” are happy supporters of ‘browsehappy’. https://www.ads-software.com/news/2005/06/browsing-happy/
Here is the thing. We initially had a page that would report this message in Internet Explorer. I was able to verify in my own version of IE (different version). I went to the supercache plugin, and turned it off, and back on. And the message went away. Then we had reports from users that they were receiving this message, and again verified that turning the plugin ‘off’ will halt this message.
To clarify, this is a built in feature of wordpress; it is not a plugin.