A workable fork would most likely require standing up new servers that would be isolated from the existing Signal user base. I think we'll see a handful of forks very shortly and it will be interesting to see how the new maintainers navigate the current landscape.
There is already a fork, it's called Session, but it's decentralized, so there's no requirement for servers to be stood up other than introducer nodes AFAIK. https://getsession.org/
This outright hostility towards third-party clients was how I knew that Signal was just a wolf wearing open-source clothing. Glad I stayed far far away.
Last I checked, the mobile clients are GPLv3 and the desktop client and server are AGPLv3. I assume a usable fork will materialize due to Signal's introduction of a token scam. Unfortunately, signal has the network effect and asking friends and family to move again will most likely fall on deaf ears. I'll move if the new project looks sustainable.