Hi there, Signal dev here. While we won't do this for you automatically, any time you link a new secondary device, we give the user the option to transfer their message history. It follows the same rules as backups: last 45 days of media for free, or all of it if you're a paid user. And even if you're not a paid user, you can request individual attachments be transferred from your primary device.

One caveat is that we don't offer this if you're re-linking an install that already has data but became unlinked. This is because we don't currently handle merging message histories. But if you cleared the data from the secondary install first, it would work. We're thinking of ways to make this smoother!

Thanks for the reply - I definitely hope you can someday get to the point where, using the backups, you can get rid of the 30 day limit for having a device online, such that the user experience is identical whether turning on a device for the first time in 29 days or the first time in 99 days - the only difference being the backend tech of where the messages are loaded from, which the user wouldn't need to know about. Or, if needed, the user getting a "this device has been offline for X days, please enter your backup password to sync all previous messages" alert.

There should really be a prompt/hint when re-linking to tell you about this caveat, I was looking at the blog post from the beginning of the year and wondering how come a feature isn't available after ~8 months.