We've not seen the last of Roy Harper / Arsenal on Arrow. Not just because the character wasn't killed when Colton Haynes stopped being a series regular (as every other character gets killed off), but because the actor tweeted out the following.
— Colton Haynes (@ColtonLHaynes) April 17, 2015
Harper is expected to return one more time before the season ends. Executive producer Marc Guggenheim says that they knew they had Haynes for season 2 and 3 which allowed them to have an endgame in mind the entire time. Haynes leaving the series as a regular does not preclude him from coming back for episodes next season and also leaves him open to appearing on The Flash and the spinoff series.
Arrow airs Wednesday at 8 PM on the CW.