We know this because it’s specifically mentioned that Superman is dead, referencing the end of the former movie; and the mid-credits scene features Bruce Wayne going to Amanda Waller (Viola Davis) in hopes for meta-human information that will aid his mission to form a superhero team, referencing what happens in the latter movie.
Superman: Dawn Of Justice and Justice League.
On the whole, David Ayer’s Suicide Squad is a movie that plays around on the outskirts of the DC Extended Universe canon, with the titular group of convicts coming together to stop the out-of-control Enchantress (Cara Delevingne), but there are hints and clues that provide audiences tips regarding where it takes place in the timeline – and that place is between Batman v.