404 Media reports (alternate site):

The FBI was able to forensically extract copies of incoming Signal messages from a defendant’s iPhone, even after the app was deleted, because copies of the content were saved in the device’s push notification database….

The news shows how forensic extraction—­when someone has physical access to a device and is able to run specialized software on it—­can yield sensitive data derived from secure messaging apps in unexpected places. Signal already has a setting that blocks message content from displaying in push notifications; the case highlights why such a feature might be important for some users to turn on.

“We learned that specifically on iPhones, if one’s settings in the Signal app allow for message notifications and previews to show up on the lock screen, [then] the iPhone will internally store those notifications/message previews in the internal memory of the device,” a supporter of the defendants who was taking notes during the trial told 404 Media.

EDITED TO ADD (4/24): Apple has patched this vulnerability.

Apple announcement:

…iPhone and iPad are the first and only consumer devices in compliance with the information assurance requirements of NATO nations. This enables iPhone and iPad to be used with classified information up to the NATO restricted level without requiring special software or settings—a level of government certification no other consumer mobile device has met.

This is out of the box, no modifications required.

Boing Boing post.

404Media is reporting that the FBI could not access a reporter’s iPhone because it had Lockdown Mode enabled:

The court record shows what devices and data the FBI was able to ultimately access, and which devices it could not, after raiding the home of the reporter, Hannah Natanson, in January as part of an investigation into leaks of classified information. It also provides rare insight into the apparent effectiveness of Lockdown Mode, or at least how effective it might be before the FBI may try other techniques to access the device.

“Because the iPhone was in Lockdown mode, CART could not extract that device,” the court record reads, referring to the FBI’s Computer Analysis Response Team, a unit focused on performing forensic analyses of seized devices. The document is written by the government, and is opposing the return of Natanson’s devices.

The FBI raided Natanson’s home as part of its investigation into government contractor Aurelio Perez-Lugones, who is charged with, among other things, retention of national defense information. The government believes Perez-Lugones was a source of Natanson’s, and provided her with various pieces of classified information. While executing a search warrant for his mobile phone, investigators reviewed Signal messages between Pere-Lugones and the reporter, the Department of Justice previously said.