"Vintage" is close to what I'm looking for. Maybe "anachronism" is too strong, but I'm looking for a word that describes something that becomes desirable since it is no longer in use, like receiving a letter in the mail.
I think "evocative" might fit what you're looking for.
adjective: evocative bringing strong images, memories, or feelings to mind.
So perhaps an evocative letter?
What about "period piece"? From google:
period piece (noun): an object or work that is set in or strongly reminiscent of an earlier historical period.