We send the messages in notifications. So the OS can keep them. Turns out Apple was doing that. There is an option you can turn on to prevent that. It is off by default.
At least on my iPhone the default is to preview messages only when unlocked [0]. This user went out of their way to show previews in a locked state which meant it was vulnerable by digital acquisition without unlock code.
“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
Doesn't indicate this is an issue when you have it set to preview when unlocked.
At least on my iPhone the default is to preview messages only when unlocked [0]. This user went out of their way to show previews in a locked state which meant it was vulnerable by digital acquisition without unlock code.
[0] https://files.catbox.moe/3gwjoy.png