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 Checklist
@HasSignalBeenHacked
I followed the steps, until it got to looking for "Notification Content". That selection does not exist on my copy of Signal.