Messages send with FCM are integrated with the Firebase Analytics functionality to track user conversion and engagement.
I had a closer look and this assertion is given no citation in Wikipedia, and the rest of the paragraph cites , which is a link to a 2014 conference about GCM, not FCM:
Yilmaz, Y. S., Aydin, B. I., & Demirbas, M. (2014, December). Google cloud messaging (GCM): An evaluation. In 2014 IEEE Global Communications Conference (pp. 2807-2812). IEEE.
Firebase was acquired in October 2014 by Google, 2 months before this conference, and the conference is about Google Cloud Messaging, not Firebase, so it seems likely this source does not mention FCM at all. However, I haven't seen the original source and there's no direct link to a recording of this conference. I don't even know how to go about chasing this reference down enough to know for sure...
Anyway, it's a really weird, obscure reference for something that should be easily verifiable elsewhere, and Firebase would be advertising as a feature. Keep in mind that Wikipedia history shows this page did not even exist until 2017, so surely there were better references to use at the time?
Here are some better references:
Also, it uses Google Analytics, not Firebase Analytics, which makes more sense now that Google owns it.
Analytics also integrates with a number of other Firebase features. For example, it automatically logs events that correspond to notification messages sent via the Notifications composer and provides reporting on the impact of each campaign.
The source for the encryption used by FCM might be fine (I wish it was more detailed about "open communication channels"), but it's behind a paywall so I can't verify it. We can look at the Firebase privacy page for the type of encryption used by Cloud Messaging: https://firebase.google.com/support/privacy#data_encryption