Something that's departed is gone, or happened in the distant past. An archaeologist might unearth artifacts from a departed era.
Anything that's past or long gone is departed, including a person who has died. When your friend talks about her dear, departed Uncle Bob, you know that Uncle Bob is no longer living. The word has been a euphemism for "dead" since at least the 16th century — the traditional wedding service once read "till death us depart," rather than today's "till death do us part."