If someone chose to use, or only could use, his relays. Or to give a reason other than being raided or broke :)

A heads up would make sense if he was providing bridges, since that would impact reachability for his users. However, if it's a normal relay then there's really no point in sending a downtime notice. Paul is right that we see hundreds of relays appear or disappear each day and, besides a few particularly huge ones, the downtime probably wouldn't be noticed.