Definition of "all but"

A young Jedi named Darth Vader, who was a pupil of mine until he turned to evil, helped the Empire hunt down and destroy the Jedi Knights. [...] Now the Jedi are all but extinct.

1977, George Lucas, Star Wars, spoken by Obi-Wan Kenobi (Alec Guinness), 20th Century Fox

I practice nightly, I try to keep ahead / This art of surfacing is all but dead

1979, The Boomtown Rats (lyrics and music), “Wind Chill Factor (Minus Zero)”, in The Fine Art of Surfacing

When the first wave of postwar immigrants arrived in Britain in the 1950s and ’60s, it was a period of rising wages, full employment, an expanding welfare state and strong trade unions. Today, Britain’s manufacturing base has all but disappeared, working-class communities have disintegrated, unions have been neutered and the welfare state has begun to crumble.

2013 September 28, Kenan Malik, “London Is Special, but Not That Special”, in New York Times, retrieved 28 September 2013

A stunning performance from the Republic of Ireland all but sealed progress to Euro 2012 as they crushed nine-man Estonia 4-0 in the first leg of the qualifying play-off tie in A Le Coq Arena in Tallinn.

2011 November 11, Rory Houston, “Estonia 0-4 Republic of Ireland”, in RTE Sport‎