I wonder if any comparable services don’t re-use the address when it’s on the service’s domain, maybe after a grace period. I use fastmail mostly for my own domains though I do have some addresses on theirs. Certainly if you let a domain expire, the registrar will sell it to someone else. Seems similar.

Someone once offered me a nice chunk of cash for one of my domains. I might have accepted it if they didn’t want the domain handed over immediately. It’s a domain I receive some email through, so would have wanted a shutdown period to prevent at least some of the email for ending up with the domain’s new owner.