Well, this came out of nowhere. Ben Affleck has shared the following video on his social media channels seemingly confirming that Deathstroke will appear in Justice League. While it's certainly possible that this is a costume test for The Batman, it's probably way too soon for that, and the fact that this recording is of a monitor makes it seem more likely that it was taken on the Justice League set.
It's possible that Affleck is just teasing fans of course, but that's clearly live-action footage of an actor suited up as Deathstroke. Whether we'll get any clarification on what this means remains to be seen, but it definitely looks like this iconic Batman villain could be part of Justice League! How he'll factor into a story revolving around Steppenwolf and Mother Boxes is unclear, but share your theories below.