Though the search for Joker's body continues, the Coast Guard reports it's extremely unlikely that Gotham's most notorious criminal survived the crash.
~ Angela Chen, reporting The Joker's fate in The Batman/Superman Movie

These are villains whose fate is truly unknown because the author is not making any more stories or the TV show, movie, or video game is cancelled. This only applies when the story was ended prematurely, rather than for cases where the ending was intentionally inconclusive.