When introducing Johnny, Mirren, and Gat in We Were Liars, Cadence says, "The family calls us four the Liars, and probably we deserve it […] Most years on the island, we've been trouble" (4.2). At first, it's not clear what they're lying about—they seem pretty normal—but during summer seventeen, small lies begin to emerge. Mirren, Johnny, and Gat lie about having gotten glazed doughnuts at Downyflake, which only sells cake doughnuts; Mirren makes up a fake boyfriend.
Finally, though, it becomes clear that Cadence is living the biggest lie of all: Harris thinks the fire that destroyed Clairmont was an accident, and that Cadence wasn't there, but the truth is, she masterminded the whole thing. Oops. As for the whole past tense component of the title? Yeah, well, since the rest of the Liars died in the fire, their little gang is decidedly a thing of the past.