I was looking into Tailscale which I thought to be complexly open source, but it turns out that their coordination server is closed source. If you want to run your own open source coordination server, Headscale is the go-to option.
This is no fault of their own (as they freely express this in their FAQ) it’s just that I had always been told by people that Tailscale was fully open source. This got me wondering what else is not as open source as people widely accept it to be?
Telegram is known for being open source, but since it isn’t, you can’t self-host it. Does that count in the sense if your question?
I am not a fan of telegram, but the clients and the protocol are open source.
Where Telegram seems to come in handy is using the bots to send alerts, e.g. from Uptime Kuma. That’s all I use it for, I presume that’s what most of us use it for?
If that’s what you are using it for, I would suggest looking into Matrix / Element. Just as easy, completely open source and self hostable. Wide variety of client choices for every OS.
The server, though, is made in pretty inefficient python, eating a lot of resources to run. There are rust, c++ and go implementations promising to be less intensive
I think for the purpose of 99% if the self hosters, which is an instance for maybe a of a couple of hundred of users, this doesnt matter at all:
https://matrix.org/ecosystem/servers/