No, Signal has not been hacked. However, we do recommend turning off showing Signal content in notifications because the content is stored in memory on device. Apparently, this memory can be retrieved if an attacker has physical access to an unlocked device and has the right tool.
https://activistchecklist.org/signal/#signal-disable-notifications

Signal Security Checklist | Digital Security Checklists for Activists
Configure Signal to protect your messages and calls
Activist ChecklistThis is also a good guide on using Signal for activism. Thanks to @micahflee
https://micahflee.com/using-signal-groups-for-activism/

Using Signal groups for activism
Things are heating up. Millions of people are taking to the streets against Trump's rising authoritarianism. Communities around the US are organizing to defend against ICE raids, to protest Israeli genocide, for mutual aid, and for other forms of fighting fascism. Signal can help people safely organize in all of
micahfleeHere's a great guide to share around for folks on how to best use Signal for security and privacy.
https://activistchecklist.org/signal

Signal Security Checklist | Digital Security Checklists for Activists
Configure signal to protect your messages and calls
Activist ChecklistNo, Signal has not been hacked. Spreading false rumors about this is dangerous. There are other ways an actor can gain access to a group chat's messages that don't involve hacking Signal. For instance, you might not be able to to fully vet everyone in a large group intended for volunteers if you are rapidly trying to build up community networks. The people who want you to think Signal is hacked are the same people who are trying to make surveillance easier.