That's a thing? If my number expires and gets reassigned to someone else, and they register for Signal, I'll get locked out of my account just like that? And they'll start getting all the messages that were addressed to me?
Importantly the safety number will change since it's a new device. If you care about stuff like this, verify the new device out of band and distrust any unexpected changes. Most people don't care and they still see a huge improvement over plain SMS.
Second, Signal and other services have implemented secondary registration requirements such as a PIN, which they will require during a new device install or at other times.
Third, you can build models or crude business logic to identify when a number no longer appears used for a period of time. Carriers do not reassign a number immediately. Assigning a number thus one user cancelled, to another user, is seldom done before a 90 day hibernation period.
I used to work at a cell provider.
Still, given that your number is used as a primary identifier, I'd avoid using it in that way for an extended amount of time. Among other things, I'm not sure if it's possible to re-register using just your phone number and PIN (but not access to SMS-OTPs on the associated phone number) in case you lose your own device, for example.
If you enabled Registration Lock, your account cannot be hijacked by sms, provided you’ve been actively using your signal account within the last week.
There’s an automated keep-alive for the case that you still have signal installed but haven’t been sending/receiving any messages.