> AFAIK, it prompts at first, maybe a few times, but then stops.
It's been re-prompting me for years. If there's a time it stops, I haven't found it.
> I am not sure how Signal backups work or that user contacts, encrypted, are backed up to the SGX enclave. Where does it say that?
You're talking about backups. I'm talking about contact-discovery, wherein the client regularly sends (hashed versions of) all the phone numbers from your contacts (if you've shared them with the app) to Signal's servers, to let you (& them!) know you're both on Signal. How else would you think the notification you get when someone in your contact list joins Signal is generated?
Signal's claim that these oft-repeated intersection operations leave no permanent records on their servers seemed (last I looked deeply) based on the SGX attestation: that your list is encrypted such that only the trusted code will process it. If Signal, or hackers, or Intel Corp, or the "Intel Community" can compromise SGX's guarantees, they can decrypt & log the full set of phone numbers uploaded.
So again, it reduces to how much you trust Intel™ SGX®.
(Also note that even if you do trust SGX, someone you've never met can, by having your phone number in their contacts, receive a notification when you join Signal. And separately from any SGX-mediated threats, a persistent attacker with privileged views of your devices' network traffic – such as via an ISP or mobile carrier – can get, via the volume & timing of traffic to and from Signal's servers, a pretty good idea of who you're talking to.)