He didn't appear, but Dick said he didn't work with him in months, so it means they worked together before.
I'm not trying to prove that Slade is a Nightwing villain, I've always seen him more as a Titans Villain. I'm pointing out that things are different.
In New 52 era, the Titans were erased, that's why they didn't had any interaction until Lazarus Contract. Still, Slade didn't really fought against them in that fake past, because of the deal he made with Dick (which included Dick training Rose, so that happened in Rebirth too).
Right now, the true past is restored. We're going to see what will happen in Infinite Frontier.