moot point

English

From moot (“assembly”) + point.

moot point (plural moot points)

An issue that is subject to, or open for, discussion or debate, to which no satisfactory answer is found; originally, one to be definitively determined by an assembly of the people.
- 1919 August, P. G. Wodehouse, “Prohibition and the Drama”, in Vanity Fair, page 21:
  In this age of moot points—some mooter than others, others possibly a shade less moot than some—perhaps the mootest point of any is, What is happening to the drama now that July 1st is behind us? This surely is a point of which the mootness cannot escape the most ivory-domed.
- 2009, Barney Hoskyns, Lowside of the Road: A Life of Tom Waits, Faber & Faber, page 155:
  Exactly which of the songs on Small Change originated in London is a moot point.
- 2014 November 12, Paul Evans, “Burning red the hawthorn brings to mind moots and magic rituals”, in The Guardian‎^[1], →ISSN:
  It is possible that a special hawthorn grew on this hill hundreds of years ago and that it was a meeting place. Hawthorns marked moots, or assemblies to decide issues of local importance and manorial courts. This one may still be a “moot point” – something arguable, undecided, contested, its original function lost generations ago.
An issue hardly worth debating because it is no longer practically applicable.
Until we rebuild downtown, whether we build more parking spaces is a moot point.

Who really owned that horse is a moot point now that it has run away.

The first usage given above is the original meaning of the phrase.
The second usage given is modern and is the meaning more commonly understood in American English, possibly because of the association with moot court.

issue that is open for debate

issue that has been rendered irrelevant