There are open groups on Telegram in which people post that they want to see person XY dead... Everyone who joins the open group can read it anyway.
They decided not to moderate this and not to delete illegal content once reported, although it's easily possible without breaking encryption.
IMO, this has not much to do with privacy.
This is like posting that we should kill someone on Reddit and nothing is happening.
I'm calling the E2E algirithm open source. That's the reason why WhatsApp is using it, too. As long as the message is correctly encrypted for E2E on the client side, it should not matter what the server is doing as far as I know.