Bit of a random question but seeing as looking it up yields no results...

What happens to expired/cancelled/written off licence plate regos? Meaning as in beyond a timeframe that they could be renewed.

Do they go back into the pool of "available" regos? Do they remain attached to the original owner (e.g. can be traced back to them?). Do they become "retired" meaning they cannot be re-used for someone else?

Is there any difference in each state?