Imagine putting a git-annex drive in a time capsule. In 20, or 50, or 100
years, you'd like its contents to be as accessible as possible to whoever
digs it up.

This is a hard problem. git-annex cannot completly solve it, but it does
its best to not contribute to the problem. Here are some aspects of the
problem:

* How are files accessed? Git-annex carefully adds minimal complexity
  to access files in a repository. Nothing needs to be done to extract
  files from the repository; they are there on disk in the usual way,
  with just some symlinks pointing at the annexed file contents.
  Neither git-annex nor git is needed to get at the file contents.
  
  (Also, git-annex provides an "uninit" command that moves everything out
  of the annex, if you should ever want to stop using it.)

* What file formats are used? Will they still be readable? To deal with
  this, it's best to stick to plain text files, and the most common
  image, sound, etc formats. Consider storing the same content in multiple
  formats.

* What filesystem is used on the drive? Will that filesystem still be
  available?

* What is the hardware interface of the drive? Will hardware still exist
  to talk to it?