Once upon a time, Justice League was going to be a two-part movie. Unfortunately, the response to Batman v Superman: Dawn of Justice (specifically from a financial standpoint as it was a very expensive production) changed Warner Bros.' plans as they balked at the idea of giving Zack Snyder that much freedom.
Two films became one, but had Zack Snyder's Justice League been released in theaters in 2017 (instead of on HBO Max next year) and succeeded, that sequel probably would have ended up happening.
Regardless, back when JL and Suicide Squad were still being developed, the idea was that the latter would lead directly into Snyder's two-part epic. As you can see below, director David Ayer recently took to Twitter to confirm that "Squad was the on-ramp for JL," a reference, perhaps, to previous claims that Enchantress was being controlled by one of the Mother Boxes.
Steppenwolf was expected to make a cameo appearance near the end of the movie too, but as Ayer notes, plans change. Still, it's hard not to imagine what could have been in a truly connected DCEU.
Check out Ayer's comments below: