I reverted one of my systems to a previous clone, installed all updates, then WhatsApp Desktop only to get the same message.
I looked into /opt/whatsapp-desktop and in main.js I found "whatsApp.window.loadURL('
https://web.whatsapp.com');". So it is a browser and it loads web.whatsapp.com and the message comes from whatsapp.
I also found references to
https://github.com/Enrico204/Whatsapp-Desktop where it reads "I'm abandoning this project".
So the package is no longer maintained or updated.
However, with the current Chrome being Version 71.0.3578.80, Chrome 36+ seems to be a loooong time ago, and Whatsapp Desktop isn't abandoned that long ago. Or it should have outdated already. I don't know, couldn't find a user agent string or what looks like it. But then I am not a script-kid...
Anyway, it is clear Whatsapp Desktop is abandoned, no longer updated or maintained and will sooner or later not do what it is supposed to do so it should be removed from the software repository.