is SimpleDB back from the dead?

My understanding is that SimpleDB has never been formally deprecated and is still available. However, AWS have not made any announcements relating to SimpleDB in over a decade (https://aws.amazon.com/blogs) and it is only available through the AWS API and not the Management Console.

It may or may not be safe depending on what you mean by that term! I don't see any scenario that it'll just vanish without notice one day, but there is the possibility (like with any other service) that AWS slowly withdraw it over a period of time. I also believe it is no longer actively worked on except for essential maintenance. DynamoDB is widely seen as the successor to SimpleDB and is probably a better choice for 99.99% of purposes, and also has a Free Tier.