We saw this during Season 3 of Arrow when the Task Force X was taken off of the board because of the impending release of David Ayer’s Suicide Squad, which eliminated our shot at seeing Harley Quinn, and the same thing has now happened again because of the Justice League post-credits scene that introduces Joe Manganiello’s version of Deathstroke into the DCEU.
So while Manu Bennett’s version of Slade got to come back for the explosive Lian Yu cliffhanger and its aftermath in Seasons 5 and 6 of Arrow, he’s apparently not coming back for the foreseeable future.
Characters in The CW’s Arrow-verse have often played second fiddle to their big screen counterparts, which means shows like Arrow have had to sit and wait for fan-favorites to become available when the movies are no longer using them anymore.