It is, but open source servers never matter anyways

I thought it was open source.

https://github.com/signalapp/Signal-Server

This doesn’t prevent them from running different software or logging requests, but we have unsealed court orders, which is better than most other services that could receive them.

The server is Foss. However, that means nothing as you still be using it. Best to not trust it and use ee2e which is what Signal does.

Signal is really a fancy version of SMS and MMS. It does require a phone number but contacts don’t need that you contact you. There are better options from a anonymity perspective but they aren’t as user friendly and popular.