I am a firm believer that there are many privacy techniques you should focus on before encrypted messaging because they will offer you much more “bang for your buck,” things like good passwords, two-factor authentication, and even encrypted email. That said, I still believe that encrypted messaging is a critical part of a well-rounded privacy and security strategy. While the vast majority of our day-to-day conversations may be benign, it can still offer a lot of insight into who we are as people – our routines, likes, and personal thoughts. This information – mundane or not – is worth protecting.
If you put the notification in unencrypted form, across google’s push notification system, it is logged in puretext. I, and everyone else knows, that messages can be encrypted. This was a warning about a very specific thing.
Law enforcement has been doing this to signal users for a while now. The default is to not show the message in a notification, but users keep turning it on, and it uses Google’s notification servers. So law enforcement, got access to people’s signal messages, by going through Google to get the notification history/logs.