It looks like Signal isn't as open source as you thought it was anymore

Eh, I don't think open source necessarily implies open development. Being able to look at and modify the source code of the software I use is the bar for "open source" for me; I don't expect every organization to do all their development out in the open. Seems like this code was made available while it was still relevant, so I don't think anyone is really at fault here.

If I can't get a Signal server running myself, or if I can't compile the Signal client and install it on my phone, what's the point of being open source?

They could release a version of the server codebase that is not the production one, and who knows? They could put on the Google Play Store/Apple App Store a version of the client compiled from different sources (and mobile builds are not reproducible), and who knows?

Also the main benefit of FOSS is the possibility to take the source code, modify it, implement new functionality, fork it, and so on. And Signal is against that. So to me it's like a proprietary software.

Telegram on the other hand it doesn't provide an open source server, but the clients are 100% open, you can compile a client from source, modify a client, make third party clients that implements the Telegram protocol with the official libraries. All of that it's useful not only for improving existing clients, but for automating stuff, there are libraries for basically every programming language to interact with Telegram. That to me means being open source, and what distinguish Telegram from all the other applications.

Both the server code and client code are open source. You're absolutely free to take them, modifly them, and do whatever you want with them. You're free to make your own fork and create your own chat service.

However, signal won't (and had not obligation to) provide your forks infrastructure. It costs money and time to maintain servers and they don't want 3rd party projects leeching their resources.

Again, your free to take their open source code and modify it and stand up your own infrastructure. If you do, you'll end up with your own chat service.

I feel like you missed the point of this discussion: the server code in fact hasn't been open source for the past year, and so one couldn't make a fork of it.