That supposedly would have led to Darkseid battling the Justice Leaguers in Justice League 2, defeating them and “leveling” Earth, and then Justice League 3 would have seen the protagonists making one last stand against Darkseid and the forces of Apokolips.
Apparently the original intention for Darkseid in Justice League, along with featuring him in the flashback battle sequence, was for him to appear in front of the movie’s heroes on the other side of a Boom Tube, presumably the same one that Steppenwolf was dragged through.