With Ben Affleck having left directing The Batman and with Matt Reeves on board writing and directing the movie, the fate of Joe Manganiello as Deathstroke is currently unknown; however, Arrow EP Marc Guggenheim might have offered a big clue.
It's known WB puts the kibosh on DC TV characters whenever they are going to appear in their movies, such as when the Suicide Squad movie came out, Arrow saw various characters leave the series including Amanda Waller and Deadshot.
Likewise, when Joe Manganiello was set to appear in Affleck's Batman and possibly the Justice League movie, Arrow saw Manu Bennett depart.
Now Marc Guggenheim offers up that Joe Manganiello might be done as Deathstroke as he fills in Collider (via CB) that something changed at the end of the year that let them use Deathstroke again. Obviously that something means Matt Reeves taking over Batman.
“It was a function of - you know, DC control these characters. We went through a period where DC said, ‘We’ve got plans for Deathstroke that don’t include Arrow.’ That changed at the end of this year,” Guggenheim said. “I went to DC and said, ‘Look, I know you have plans for him and that’s all cool, but you’ve got to give us this one. We can’t go to Lian Yu and not see Slade Wilson. [Arrow] has to operate within the departments of DC.”
Now bear in mind it could be possible that DC let Arrow use Deathstroke because the Batman movie may simply be delayed, and that Joe Manganiello will still play Deathstroke.