The thing has been that historically even though Amazon would "depreciate" a service they kept it running and supported.
"Amazon SimpleDB (N. Virginia) Service is operating normally" is their current status page, but their marketing materials doesn't show any SDB stuff. So if you had some old workload ticking over - you could just leave it usually.
I actually used SimpleDB in a light use case wildly past the point at which it disappeared from all marketing. I kept on expecting an email like the EC2-classic one, but it never came even though simpleDB didn't show up anywhere really