Well, they threatened LibreSignal with legal action, both for accessing their service and for using the Signal name, demanding that any third party client runs their own entire separate server network. Sounds pretty much like a ban.
They (the forkers) were stupid to induce brand confusion like that.
Signal is GPL free software; you can fork it and release it with any API URLs you like in the source or binary. It is perfectly legal to fork Signal and remove the user-hostile expiration timer, the image scaling enshittifier that happens on upload, and leave the signal.org API URLs completely unchanged. You just can't call it Signal (or LibreSignal, or BetterSignal, or ExtraSignal, or whatever) when distributing it.
The API ToS is what applies to end users who connect to the API. It says nothing about what software you are allowed to use to do so.
Signal would prefer to pretend that they get to choose what tools their users get to use to consume their service. Unfortunately for them (but good for us), they don't. It's the web and such a restriction would be insane.
Don't confuse software and services. They aren't the same.
I understand that federation and defined protocols that third parties can develop clients for are great and important ideas, but unfortunately they no longer have a place in the modern world. Even less of a place for an organization the size of ours. Everyone outside the FOSS community seems to know it, but it took actually building the service for me to come to the same understanding, so I don't expect you to believe me. [0]
I push back when anyone recommends Signal because it is fundamentally not an open network.
Both absolutly do hamper fast evolution of a product because they require a LOT of coordination and consensus, both of which are incredibly expensive and time consuming.
With a non-federated and unstable protocol, they are free to update and deprecated whatever they like and on a fast timescale. That is impossible with a federated stable protocol.
This has nothing to do with FOSS.
XMPP is a good example of a protocol that got fragmented to death by many many different and incompatible extensions, ultimately making it too fragmented and too slow to adopt to new requirements.
Third party clients really should be fully allowed too (even though they're not actually being banned so technically they're being condoned)
https://en.wikipedia.org/wiki/Computer_Fraud_and_Abuse_Act
Sadly, they do get to choose what is considered "authorised" use of their service :(