As it turns out, Deathstroke was written out of the show because DC had bigger plans for the character to show up in the DCEU’s The Batman when Ben Affleck was still on board as director.
Whatever the case may be, it is almost certain that we will not see Deathstroke in The Batman and becoming more and more likely that Ben Affleck may be out as well.
But apparently those big screen plans for Deathstroke are over as Season 6 of Arrow is set to bring the character back.
Ben Affleck teased that Deathstroke would be in The Batman and shortly after, it was confirmed that Joe Manganiello (True Blood) would be playing the assassin in Affleck’s version of The Batman. Obviously, a lot has changed for the project in a year with Affleck’s script tossed in the garbage and rewritten by new director, Matt Reeves.