Signal (just iOS strangely; Android has had exporting for years) has been completely anti-backup basically since it's inception. That's their prerogative, but I think it's something that needs to be made clear, front and center, before anyone starts using Signal (or asking anyone else to use Signal). Signal is the type of messaging that needs to be treated as a black hole.

Here's issue #905 from 2015, a request for transferring messages when moving phones, closed the same day: https://github.com/signalapp/Signal-iOS/issues/905

Similarly, #967 https://github.com/signalapp/Signal-iOS/issues/967

And #2542 (2017, when I was first looking for transferring my Signal to a new phone) https://github.com/signalapp/Signal-iOS/issues/2542

That issue was locked and I was told to create a thread on their forum instead: https://whispersystems.discoursehosting.net/t/ios-backup-kee... - 3.5 years and 400+ messages later, backup functionality still doesn't exist for iOS, and the best solution for archiving your old messages requires jailbreaking and extracting encryption keys from your keychain: https://cight.co/backup-signal-ios-jailbreak/

NOTE: last year, Signal introduced functionality for transferring messages to a new device, but that won't help you from data corruption or losing your device since backups and export are still not supported: https://signal.org/blog/ios-device-transfer/