The DC Extended Universe began in 2013 with Man Of Steel and currently consists of nine movies, the most recent of which being Wonder Woman 1984, which came out last month. There are at least six DCEU movies in production, including The Flash, Black Adam, and this year's The Suicide Squad, to say nothing of Zack Snyder's Justice League director's cut and various TV series for HBO Max.
With so many projects in many different hands, it's inevitable that whole characters would disappear, get changed around, or be forgotten entirely. Some had movies that were dropped and others may still turn up in sequels, but there are multiple characters who simply disappeared from the DC movie universe.
Kara Zor-El/Supergirl (Man Of Steel)
A Supergirl spin-off of Man of Steel in the DCEU has been rumored for several years. This is a shame, not just because the Supergirl TV series starring Melissa Benoist is coming to an end in 2021, but there have already been steps to tease Kara Zor-El in the DCEU.
In Man of Steel, one of the pods in the Kryptonian scout ship was clearly empty. While Zack Snyder eventually debunked the theory that this was for Kara, the movie's prequel comic written by David Goyer (who wrote Man of Steel) and Snyder himself showed the theory to be correct. Either way, this thread has been forgotten for now.
Robin (Batman V Superman)
One of the biggest unresolved teases in the whole DCEU is the Robin suit hanging in the Batcave in Batman V Superman. The suit is graffitied, seemingly by the Joker, which suggested the "Death in the Family" story happened to Ben Affleck's Batman at some point, although not necessarily with Jason Todd dying.
Suicide Squad added more to this tease by having the on-screen bio for Margot Robbie's Harley Quinn implicate her in the death of Robin. Fans may never find out the truth, although it's possible that the Nightwing movie in pre-production may shed some light on it.
General Swanwick/Martian Manhunter (Man Of Steel/Batman V Superman)
The character of General Swanwick appeared in both Man of Steel and Batman V Superman, and was one of the more amiable military personnel towards Superman and Lois Lane, despite being tough on the surface. Swanwick was unceremoniously dropped from Justice League, which was a shame because according to director Zack Snyder he is actually a major DC Comics character in disguise: J'onn J'onzz, the Martian Manhunter.
Snyder never got to enact his plans for Swanwick in the DCEU, but it's possible fans may still get the twist with the character in the Justice League Snyder Cut coming this year.
Green Lantern (Justice League)
The run-up to Justice League kicked off before even Batman V Superman was released, with the first look at Jason Momoa's Aquaman bearing the words "Unite The Seven." While this seemingly referred to the traditional seven members of the Justice League, one hero was essentially absent from the movies: Green Lantern.
While Justice League did briefly show an alien Lantern in the ancient battle with Steppenwolf, and the movie's trailer mentioned the Earth having "no Lanterns," the team is still missing its seventh member and there's no news on the Green Lantern Corps movie.
Faora (Man Of Steel)
Audiences got a good look at the fate of General Zod in Man of Steel, as Superman was forced to kill him rather than let him murder any more people. What is unclear, however, is what happened to the other Kryptonians, in particular Zod's sadistic second-in-command, Faora.
Faora is even crueler than Zod, who at least was fighting to preserve Krypton's heritage. Faora seemed to just enjoy hurting people and frequently defeated Superman in direct combat. She was cast into the Phantom Zone with Zod's ship, which means she's still alive, a fact completely forgotten by the DCEU.
Slade Wilson/Deathstroke (Justice League)
One of the most popular DC villains is Slade Wilson, otherwise known as the deadly mercenary Deathstroke. Over the last few years, Slade has made major appearances in Arrow, Batman Arkham Knight, Titans, Injustice, and the Teen Titans Go movie.
Joe Manganiello's very brief cameo appearance as Deathstroke in the post-credits scene for Justice League, alongside Jesse Eisenberg's Lex Luthor, was meant to tease either Justice League 2 or The Batman. With the former canceled and the latter no longer part of the DCEU, Slade Wilson has to stay on the sidelines.
Etta Candy (Wonder Woman/Wonder Woman 1984)
After leaving Themyscira with Steve Trevor, and especially following the death of Steve at the end of Wonder Woman, Diana Prince and Steve's former secretary Etta Candy became close friends and went on multiple adventures.
Wonder Woman 1984 centers on Diana's growing detachment from humanity and her unwillingness to form new relationships, and yet the fate of her first friend Etta is glossed over, with just a single photo of her as an old woman in front of New York to hint at what happened to her.
Darkseid (Justice League)
The ultimate evil of the DC Universe, Darkseid is the lord of Apokolips, the master of the parademon army, and the boss of Steppenwolf. He was namechecked by Steppenwolf in Justice League, who suggested that his invasion of Earth was in the name of Darkseid. Otherwise he plays no part in the story.
Darkseid was planned to be the main adversary in Justice League Part 2, but that movie's cancelation has left him in limbo. Nevertheless, Darkseid will have a role in the Justice League Snyder Cut when that drops and is supposed to be the main villain in the New Gods movie that was announced in 2018.
Tatsu Yamashiro/Katana (Suicide Squad)
Katana is an expert swordswoman, a samurai warrior, and the wielder of the Soultaker sword which can steal the souls of its victims. As her appearance in Suicide Squad proved, Tatsu Yamashiro is a badass, and more than capable of keeping Task Force X in line.
It's sad, then, that she has been mostly forgotten about by the DCEU. There is no resolution or epilogue for Katana in the movie, unlike the rest of the Squad, and she has not been announced as returning for semi-sequel The Suicide Squad.
The Joker (Suicide Squad/Birds Of Prey)
Suicide Squad features the first DCEU appearance of the Clown Prince of Crime after he was teased in Batman V Superman, played by Jared Leto. While Joaquin Phoenix would star in the non-DCEU Joker, other movies with Leto's Joker were planned, including one centered around him and Margot Robbie's Harley Quinn.
However, while Joker would get mentioned a lot in Birds of Prey, the character does not physically appear. Therefore Batman's nemesis is still out there, waiting to return to prey on Gotham.