It is possible to defend against them. Maybe not on your phone though.
So having a signal for switching mediums is something that I feel indicates thinking in the wrong direction.
It's not for everyone. I grew up with code phrases. My mom knew that if I said "I love you" to send in the cavalry. We had similar processes in the military. If I answered the phone a particular way they knew the remote site was under siege.
Everyone knows you talk to your parents, but code phrases are not a way to get privacy.
Fundamentally I agree with you but people will stay on the platforms where their friends are. To change that the platform would have to do something really bad such as forcing age checks and even then I think many will just put up with it to stay connected to their friends.