Here you can find the documentation for versions of OpenMDAO from 2.0 to 3.9.2.
latest    
Released on June 2, 20233.9.2    [Release Notes]
Released on May 14, 20213.9.1    [Release Notes]
Released on May 4, 20213.9.0    [Release Notes]
Released on April 28, 20213.8.0    [Release Notes]
Released on March 16, 20213.7.0    [Release Notes]
Released on February 12, 20213.6.0    [Release Notes]
Released on January 15, 20213.5.0    [Release Notes]
Released on December 4, 20203.4.1    [Release Notes]
Released on November 13, 20203.4.0    [Release Notes]
Released on October 2, 20203.3.0    [Release Notes]
Released on September 4, 20203.2.1    [Release Notes]
Released on August 7, 20203.2.0    [Release Notes]
Released on July 21, 20203.1.1    [Release Notes]
Released on June 12, 20203.1.0    [Release Notes]
Released on May 1, 20203.0.0    [Release Notes]
Released on March 5, 20202.10.0    [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