be a moot point
To be a topic that can no longer be questioned or debated. Whether or not he's the best person for the job is a moot point now that he's tenured.
be a moot ˈpoint/ˈquestion
be a subject that people disagree on or are uncertain about: It’s a moot point whether women or men make better drivers.A moot was a group of people who met to discuss questions of local or national law during the Anglo-Saxon period. A moot point was a question of law discussed at this meeting.