Those crossovers were with Titans and Teen Titans, which makes sense since Slade's debut was in New Teen Titans #2 in 1980. It's actually the Titans books that were slave to Deathstroke's ongoing plot. The first one ended with Wallace West quitting and joining Deathstroke's Defiance team. TT got him back when that story ended and he survived the roster change later on. The second one ends with Robin thinking he'd killed Slade. Of course, it was obvious in advance that he was faking it, else Deathstroke would've ended it's run there and then.