Not sure what part of my comment amused you so much.
An IM platform server can be open sourced. Just like any kind of software.
It's just a matter of publishing your code and, preferably making it possible to verify that the service your users are connecting to is build using the same published code.
How could you possibly verify what code they are running server-side?
Typically, the way it goes is that you implement e2ee such that even a fully compromised server cannot read the clients messages, publish the client's source code, and build it yourself or use reproducible builds. That ladt part is where you can criticize Signal. Whether they publish the server code is mostly irrelevant unless you want to run a separate messenger infrastructure.
> unless you want to run a separate messenger infrastructure.
Or if you S2S federate with the upstream server. Which is a core differentiator of XMPP and Matrix. Signal server(s) notably supported proper federation during their initial growth-phase but famously closed it off ("The ecosystem is moving").
Similar story as Google [Chat/Talk/Hangouts], which did federate over XMPP before they closed that down years ago.
An IM platform server can be open sourced. Just like any kind of software.
It's just a matter of publishing your code and, preferably making it possible to verify that the service your users are connecting to is build using the same published code.