There's no technical reason why they couldn't sync messages, but there is an attack if they do. Basically, someone could set up a new device with your account and grab all of your past messages. Since Signal doesn't sync messages, they'll only get messages going forward.
It should certainly be an option to sync, but I don't think it should be on by default (or at least it should ask on setup).
Most people weren't following the Chagos Islands news, and I doubt most people with
.io
names bothered to check any notifications here. A lot of people just pick them up and set them to auto-renew and generally don't think about it again. Those people won't be impacted today, but they will be once the domains get transitioned away, and it'll be a rude awakening for a lot of people.The simple solution is to not buy country TLDs unless you live in that country or something.