I'm with you on this, I strongly recall there was some sort of not fully open source portion of Signal at least at one point in time.

Edit: ya, they weren't updating server for awhile, so while there is an open source server, they definitely weren't running that code for awhile, and may not be running it today. Granted since the decryption happens client side, it shouldn't matter what the server does to some extent.

https://www.androidpolice.com/2021/04/06/it-looks-like-signal-isnt-as-open-source-as-you-thought-it-was-anymore/

There was a period where they didn't push changes to the repo, but all the code was released afterwards and it's been getting regular updates ever since. But it also doesn't matter at all, since the Signal client is designed in a way that avoids putting trust in the server. Signal servers could literally be run by the NSA and it wouldn't matter, as everything is fully end-to-end encrypted, including metadata. The Signal protocol was also updated to use post-quantum cryptography in 2023.