Signal typically delay releasing the server source code, so the latest version of the server is not open source. In one case it took them almost a year (no public commits between 20 April 2020 and 6 April 2021).
https://github.com/signalapp/Signal-Android/issues/11101#iss...
Among the official reasons given was staying ahead of spammers. In this instance it was also speculated that the payment function which they were building into the server was to remain secret.
https://github.com/signalapp/Signal-Android/issues/11101#iss...