John Stewart's Green Lantern was originally supposed to appear at the end of Zack Snyder's Justice League but Warner Bros. decided against it, wanting to save the character for another project. John Stewart was introduced in the pages of Green Lantern vol 2. #87. Since his introduction has become a major player in the pages of DC Comics and to an entire generation is the most well-known Green Lantern due to his presence in the animated series Justice League and Justice League Unlimited.
Green Lantern has been a founding member of every incarnation of the Justice League since the comic premiered, so the lack of Green Lantern on the team in the film raised a lot of eyebrows. While a Green Lantern was not planned to be part of the team in the film, one does appear in the flashback holding off Darkseid's forces. Snyder even revealed he had plans to bring Ryan Reynolds back as Hal Jordan at one point but those plans never materialized.
During an interview with Vanity Fair, Zack Snyder explained that his original plan for the ending of Justice League was for John Stewart's Green Lantern to meet Bruce Wayne, but Warner Bros. vetoed the decision. The studio didn't want John Stewart in the film because they had plans to introduce him in another project. The character was instead swapped out for Martian Manhunter in the final scene. Snyder elaborated:
“We shot a version of this scene with Green Lantern, but the studio really fought me and said, ‘We really don’t want you to do Green Lantern...So I made a deal with them, and they let me do this [instead]...They were like, ‘We have plans for John Stewart and we want to do our own announcement.’ So I said all right, I’ll give you that. So [Martian Manhunter] was the compromise."
As for what Warner Bros. was saving John Stewart for, it could be a number of different projects. The most obvious is the Green Lantern television series being planned for HBO Max. That series already has confirmed the appearance of Alan Scott, Guy Gardner, Simon Baz, and Jessica Cruz, so it stands to reason that John Stewart, one of the most popular Green Lanterns, could appear. But the fact that John Stewart has not been announced for the series suggests Warner Bros. could still have plans for a future Green Lantern Corps movie.
John Stewart could also appear in the Arrowverse, though that likely wouldn't affect his ability to appear in films or HBO Max shows. David Ramsey's character John Diggle was teased as being the Arrowverse incarnation of John Stewart. When The Flash of Earth-90 (John Wesley Shipp) sees Diggle, he comments on him not wearing his ring, and in the final episode of Arrow, Diggle approaches a meteorite site that glows green. Ramsey is currently set to guest star on various episodes in the Arrowverse including The Flash, Supergirl, Superman & Lois, Batwoman, and DC's Legends of Tomorrow. In Legends, he will not appear as John Diggle - instead, he'll be playing a mystery character, with many indications of it being Green Lantern. While Green Lantern may not show up in Zack Snyder's Justice League, fans hopefully won't have to wait long for the character to appear in live-action.
Source: Vanity Fair