Here you can find the documentation for the 2.x versions of OpenMDAO.
2.10.0    [Release Notes]
Released on February 27, 20202.9.1    [Release Notes]
Released on October 8, 20192.9.0    [Release Notes]
Released on October 1, 20192.8.0    [Release Notes]
Released on June 28, 20192.7.1    [Release Notes]
Released on May 31, 20192.7.0    [Release Notes]
Released on May 28, 20192.6.0    [Release Notes]
Released on February 22, 20192.5.0    [Release Notes]
Released on November 1, 20182.4.0    [Release Notes]
Released on August 1, 20182.3.1    [Release Notes]
Released on June 20, 20182.3.0    [Release Notes]
Released on June 12, 20182.2.1    [Release Notes]
Released on April 2, 20182.2.0    [Release Notes]
Released on February 9, 20182.1.0    [Release Notes]
Released on December 8, 20172.0.2    [Release Notes]
Released on October 25, 20172.0.0    [Release Notes]
Released on October 25, 2017