I'm sure they would like for it to be fully functional, but if they do, it will take time to develop, as they have to find every scenario in the game that silence would affect. That's a fairly large endeavour if you think about it.
I don't think that it's larger than all the animals / corpses we can speak to or thoughts we can read.
I disagree on the animal comparison. Every animal created has already had all of its lines planned out, as Larian is extremely fond and acutely aware of the fact that speak with animals exists. So while it is a large task, Larian brings every animal in with the framework for it ready to go, meaning they don't have to go back and retroactively fill in as many gaps from immersion breaking circumstances.
I agree with your other examples, but from my experience those also seem far from being fully implemented. They are also extensive tasks, so that's understandable, but like I said will take time to fully flesh out.