elasticsearch/elasticsearch — pull request #8850 by s1monw (2014-12-09 15:07:53+00:00)
Title: [CORE] Drop support for state written pre 0.90

Today we have several upgrade methods that can read state written
pre 0.90 or even pre 0.19. Version 2.0 should not support these state
formats. Users of these version should upgrade to a 1.x or 0.90.x version
first.