https://www.bleepingcomputer.com/news/security/signal-will-let-you-sync-old-messages-when-linking-new-devices/

Signal is finally adding a new feature that allows users to synchronize their old message history from their primary iOS or Android devices to newly linked devices like desktops and iPads.

The transfer process is fully end-to-end encrypted, ensuring privacy and data security. It will employ a QR code verification step to ensure the action is authorized.