He was later left in a jail cell on Lian Yu at the end of Season 2, which is where he remained until Oliver recruited him to defeat Prometheus in Season 5.
And since Oliver gave Slade some new information about his son, that will almost definitely serve as a narrative avenue.
Season 6 will possibly see the two of them return to being quasi-allies, further exploring their weird mentor-pupil relationship.
Deathstroke was a highlight of the earlier years, and is widely considered the show’s greatest antagonist.