alt Hacker NewsSignal still doesn't allow you to backup/export chat history on iOS into an open format? I think now they have some bullshit proprietary paid cloud storage solution (why not let me use the cloud I already pay for?), but for years they haven't had any solution for iOS at all.
Last time I had to reinstall my phone I ended up having to use & fix some Github project that simulated Signal's transfer protocol to simulate a target device to export my data.
I then deleted Signal and migrated to iMessage/WhatsApp and called it a day.
Any time an app has bizarre functionality gap on iOS, I assume it's because of Apple's anti-consumer bullshit app restrictions.
No idea if that's actually what's going on, but Apple thinks of their devices as appliances and hates when apps offer pro-customer features.