What do you mean by "does not depend on tracking"? Do you mean that you can conceive of a way to implement the service as it appears to the user, without tracking? Because Facebook can certainly inject tracking into their chat app. Given Facebook, I assume it's already the case that if I type "let's get married" and then erase those words and never send them, that Facebook's going to start showing me advertisements for how to pop the question.

You assume that, I assume that, these thoughts are a few digits shy of the decimal point of the percentage of people in the world who understand this. It's not commonly known that Facebook track your every conscious and unconscious interactions with their sites, apps, and every other site that has one of their tracking pixels/logins. It's even less commonly known that they gather data from numerous data brokers to enhance the data they have on you.

Tracking is not required to deliver a text message between two contacts who approved each other.

Facebook might declare otherwise, but since SMS and IRC both stand as counterexamples of successful text chat services that do not inherently depend on tracking, they would likely lose their case.

Define "doesn't depend on" because I could easily make the argument that Messenger's chat does require tracking location for example to increase security and to fight spam. By not opting in to tracking of location, you degrading the quality of the service and affecting other people.

i.e. Without this tracking, the quality of Messenger is affected, and therefore, by definition, the service cannot be provided without it.

That's fine, but then FB can only use that location for increasing security and fighting spam. Slurping information based on an excuse and then using it for something else is explicitly prohibited by the GDPR.