For anyone who still had doubts, Justice League director Zack Snyder confirmed on Vero today that an image seen in the film's trailers and then eliminated from the final cut of the movie features a digital projection of Superman, not Supergirl or Green Lantern as some had speculated. The image, in which a cross-armed Bruce Wayne (Ben Affleck) looks sadly at a digital image, always seemed most likely to be Superman, just like the scene in which Alfred (Jeremy Irons) recognized Superman coming into the Batcave, one of the only deleted scenes to be officially shared by Warner Bros. following the film's release.
Still, there were rumors that it was a Green Lantern (based on fans thinking they saw something reflected in Alfred's glasses) or that it was Supergirl (since the transparency and shading of the costume made some fans think Superman's legs looked flesh-colored, and his moving/flowing cape made it appear shorter). Snyder put those guesses to rest when asked about it earlier.
You can see a screenshot below, via Reddit:
Fans also noted that behind Bruce, you can see the van that the Justice League uses when they go to the cemetery in Smallville to steal Superman's body for the revival process.
Justice League Part One and Part Two were announced at the same time, with filmmaker Zack Snyder supposedly filming them back to back. That did not last long, though. Snyder eventually, famously, either left Justice League or was forced out shortly after the death of his daughter. But even before that, a set visit during production on the film included quotes that indicated that Part Two was not guaranteed to happen, and might not happen with Snyder even if it did. Conventional wisdom says that before he exited the movie, the plan was to build a trilogy of films, but even at its most bullish, Warner Bros. only announced the two before things started to change.
When Justice League was released in 2017, with Snyder as the sole credited director of the movie but everyone knowing that Joss Whedon had overseen significant reshoots and dramatically cut the film back from its original runtime to meet studio demands, the film was relatively well received -- as long as the bar you are using for that statement is the one set by other DC movies, which up to that point had been largely hated by critics and divisive among fans.
Its poor box office performance cemented what many fans already expected: Snyder was done with DC films for the foreseeable future, and Justice League Part Two was shelved indefinitely. It seems that the best, if not only, chance to see new, Snyder-directed DC content for the foreseeable future would be if Warners releases a the Snyder cut of Justice League -- regardless of how long a shot that might be.