Justice League's behind the scenes situation was already a source of controversy prior to the movie arriving in theaters, but nobody could predict the true escalation the drama would see in the months following release. When Joss Whedon stepped in after Zack Snyder's departure, fans were worried that the movie would suffer from a tonal schism, poorly integrated humor, and lack cohesion with previous entries, but the cast and crew swore Justice League was still Zack Snyder's movie and would adhere to his vision.
Needless to say, when the movie arrived that wasn't the case. A significant portion of the movie is clearly the result of Joss Whedon led reshoots, storylines from the previous movies are retconned or abandoned, a significant (more than usual) number of popular trailer moments are missing from the final product, and even the movie's color grading lacks the depth and contrast fans expect from Zack Snyder's aesthetic.
As a result, fans immediately began clamoring for the "Snyder Cut" of Justice League, assuming that the original director had done enough work to have an intact and usable version of the movie completed, or close enough, to polish up for release, blaming the film's low critical score and embarrassing box office turnout on Warner Bros. meddling and tone alterations. While there are a number of other factors that can be blamed for the disappointing critical evaluation and ticket sales, the movie's low opening - before people were aware of the extent of the changes - suggests it's only a minor factor - although the movie's poor legs could definitely be related.
It wasn't long before a petition for the release of Snyder's original cut of the movie (including the original score from Junkie XL) made the rounds, racking up whopping 170k signatures to date. Despite the enthusiasm, a number of skeptics have claimed that Snyder never finished more than an early assembly cut, which would be so unwieldy and lacking in polish or completed visual or audio effects that it wouldn't even be enjoyable to watch.
Even so, others have claimed to have verified that a 'relevant' version of Snyder's cut does in fact exist, and could conceivably be completed for release. Barring an official announcement from Warner Bros., Snyder himself, or a massive leak, it'll continue to be a hotly debated issue, but a simple review of some of the already known production information shows that Snyder's cut of Justice League was in a fairly late stage of production when he left the project. The question is not whether the cut exists, but how much work was left and will it ever be released to the public?