I have an old device which I have saved because the messages on it are emotionally important to me, but that device is too old to transfer to my new device.

Since iOS 12 or so, iPhone has a built in files app. Every app can integrate with that. So when I create a file (let’s say chat backup) in app A, I can put it in the files app. Then in app B (or app A on a new phone) I can easily open that file from the same files app.

iOS has had a Files app for years, locally. You could easily export an encrypted .zip from Signal and save it locally, just like how Signal on Android saves it to your internal storage.

For some reason, the Signal devs won’t even acknowledge this possibility and continue to say “we can’t enable iCloud backup” - which is fair enough, but nobody is asking for that and they’re simply putting their fingers in their ears.