Over time, support for more and more boards gets added to U-Boot -
  while other board support code dies a silent death caused by
  negligence in combination with ordinary bitrot.  Sometimes this goes
  by unnoticed, but often build errors will result.  If nobody cares any
  more to resolve such problems, then the code is really dead and will

  be removed from the U-Boot source tree.  The remainders rest in peace

  in the imperishable depths of the git history.  Please use the tools
  git provides to read through this history.  A common example would be:
  $ git log -p --follow -- board/technexion/twister
  to see the history and changes made to the Technexion "twister" board
  from introduction to removal.