There was (and still is, so far as I know) no upfront warning to users that if they don't first sync with a desktop client, and their phone gets lost or stolen, their iTunes backups do not (unlike most iPhone applications) contain their Signal chats. And furthermore, there's no way to export those chats in backup format from an old phone.
(You can transfer, but the transfer deletes the data from the original source, which is extremely foolish and dangerous IMO, and anyways isn't a proper export accessible from other applications. Furthermore, so far as I know there's no support for transferring from very old versions of the Signal client.)
This has been a critical bug for years [1], it's one of the most complained about issues, and Signal has done (and intends to do) absolutely nothing to fix it. It is absolutely unacceptable to have our own data held hostage by them in this way, especially without any upfront warning.
[1] https://community.signalusers.org/t/ios-backup-keeping-messa...
Most likely this is just one of the walls of the walled garden.