[B-Con]

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.

[pseudalopex]

No. The Signal developers opted out of iOS's backup and export features.

[fph]

I have no idea why, but I would bet it's because it was sending stuff to Apple unencrypted.

[Nextgrid]

It's because Signal has some unhealthy obsession with "security" and does not want to recipient of the communication to ever be able to export messages in plain text.