For open source messengers, you can check whether they actually encrypt your messages and whether the server has access to your encryption keys but what about WhatsApp? Since it's not open source, you can't be sure that the encryption keys aren't sent to the server, right? Has there been a case where a government was able to access WhatsApp chats without reading them from the phone itself?
It wouldn’t surprise me if WhatsApp’s model on this is what the UK government were thinking of with the Online Safety Bill when they tried to enforce a back door in encrypted messengers.
It’s incredible just how much more interesting metadata can be than the actual message contents.
Explaining this to people when they ask why I don’t use WhatsApp is pretty difficult though.
I wouldn’t feel comfortable if I found out that what I thought was just a casual walk down the street mindlessly chatting with a friend turned out to also involve a third party neither of us were aware of tracking all of our movements.