According to the filing, Flynn has been cooperating in “several ongoing investigations.” 

His cooperation in one criminal investigation has been fully redacted. Other descriptions of his cooperation with Mueller’s team are also redacted, but they acknowledge he assisted government investigators on a “range of issues, including interactions between individuals in the Presidential Transition Team and Russia.”

Robert Mueller files sentencing memo on Trump ex-national security adviser Michael Flynn – CBS News

Robert Mueller did not come to play.