With 60 years of history behind the series, Doctor Who producers have done an admirable job of blending elements of the classic era with this modern incarnation. However, since Carole Ann Ford left the show in 1964, the character of Susan Foreman has yet to return to the Whoniverse. This is despite other classic companions coming back for a second act. But thanks to bigeneration, perhaps Susan Foreman's absence can finally be explained.
Source：https://www.cbr.com/doctor-who-bigeneration-susan-foreman-carole-ann-ford/
