There are no backups available on the iPhone/iPad app, only a device-to-device transfer while setting up a new device assuming your previous device and new device are both iPhones/iPads. This is despite support for apps storing files to the filesystem that was added some years ago now, and many other apps on those platforms supporting backups of custom file formats (or JSON, etc.).

https://support.signal.org/hc/en-us/articles/360007059752-Ba...

I'm currently facing this issue.

The process to transfer the history is to scan a QR code displayed on the new phone by the app on the old phone.

Well, the camera on my old iPhone is broken. The phone has 3 other working cameras, but I cannot switch which one the app uses…