That's it. That's all he posted is a picture. There is a rumor going around right now that there is a Deathstroke movie in the works but it hasn't been confirmed in any capacity. Joe Manganiello was cast as Deathstroke back in the summer of last year and in August then-director Ben Affleck shared some test footage with us. Then Affleck dropped out as director of The Batman and Matt Reeves came on. Then Reeves decided to throw out the entire script that Affleck wrote thus calling into question whether or not Manganiello was still cast in the role. If the script was thrown out then there was a chance Reeves could decide to do a different villain.
— Joe Manganiello (@JoeManganiello) October 27, 2017
Then the rumors about a solo movie started and this picture was posted. That's all that is, a picture of the mask, and doesn't confirm anything. While this Deathstroke movie could be real and might happen that mask is not confirmation. It could mean Manganiello is still the villain in The Batman despite the script re-writes but to jump on that single picture and get "there's a solo movie" is a stretch.
If and when Warner Bros comes out and confirms or denies that this movie is happening then we can read into a picture of a mask. Until then we are Jon Snow; we know nothing.