It has been seemingly an open secret that developer Rocksteady Games are working on a Superman game, and it's been rumored to be in development for several years now. The most recent leak comes from the popular image board 4chan, which has been known for housing legitimate leaks in the past. It's also been the home of thousands of fake ones, so these types of posts always have to be taken with a heavy grain of salt, even when they look believable.
Rocksteady Games are best known for their work on the Batman: Arkham series. After the caped crusader had starred in a number of underwhelming titles, the studio managed to use him in one of the best games of 2009 when they released Batman: Arkham Asylum. Since then, they've released two sequels and the baton has reportedly been passed to WB Games Montréal to work on new Batman-themed games, though it's unclear what those might be. And it's possible that Rocksteady has moved onto developing a Superman game.
According to a 4chan user, Rocksteady's game's is titled Superman: World's Finest. It will be a single-player only experience in which Brainiac will serve as the main villain. It's set to be revealed during Microsoft's E3 2018 press conference, although it's likely not an exclusive title. The post also says that "other DC Heroes are involved" in the game, but it's not specified which heroes or exactly what that means. It could just mean anything from appearances from other heroes to the start of their own video game universe or other playable characters being in the open-world game.
The post also included the game's poster, but it's also why this rumor seems highly likely to be fake rather than the real deal. Upon closer inspection, Superman's costume appears to be that from 2013's Man of Steel. The name is also borrowed from the comic book World's Finest, which sees Batman and Superman teaming up. However, if they were to do a game that stars both of them, it would make more sense for Batman to also appear in the branding. No previous rumors have indicated that it would feature both of DC's major heroes, so that just seems off in general.
So, while it's highly likely that Rocksteady is working on a Superman game for Warner Bros. Interactive Entertainment, this leak doesn't seem to be the main deal. It may even get some details right, such as it being revealed during Microsoft's E3 press conference, but it seems more like a series of educated guesses rather than an exclusive scoop.