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 still works (with a few minor updates).
I wouldn’t recommended using old software if you care about security
Maybe so, but in this case the point was that the protocol used by WhatsApp hasn’t changed in that time and it’s still what they describe in their security whitepaper. If you want to use that software as is or maybe reimplement it based on that is up to you.