Forward secrecy means that compromise of a long-term key doesn't reveal past messages. Signal does provide that. It's a different category of attack if someone gets access to your device and sees your chat history, or tricks you to link an untrusted device.
Export is possible in any client, at the very least with screenshots or by writing the message history on a piece of paper, but that's not related to forward secrecy. Disappearing messages is the solution when you don't want your message history potentially seen by anyone later.
You can use a VPN to hide your geolocation from Signal (which you should do on Nostr as well if you need privacy), but it's true that they can connect your messages to your phone number, see when and probably with whom you're chatting. Not surprised if intelligence agencies are mining that data.
With Nostr we're at least over the phone number part, there's no single service that sees all traffic and there are different ways to fix the metadata issue — and do forward & backward secrecy when desired.