There are dozens of us!

Signal uses the same E2E encryption as WhatsApp and is also not Open-Source. One could argue that not being owned by Meta makes it inherently more private, to which I would somewhat agree.

The most private Messaging-App would be some kind of federated Chat, like Jabber or Matrix, that you or a collection of family/friends can self-host.

Unfortunately, nobody uses those ¯\(ツ)/¯

I think the lack of a karma equivalent, and thus karma farming, results in much more thought out and unique posts/comments.