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.