For Oliver Queen's funeral, the Arrow series finale brought back a long list of characters tied to Oliver's past, but one glaring omission was Manu Bennett's Deathstroke. Introduced in the flashbacks of Arrow season 1, Deathstroke was the main antagonist of season 2. He also had guest appearances in seasons 3, 5, and 6. He hasn't been on the show since Arrow launched a two-part story in season 6 that centered around Slade's search for his son.
After Oliver died in "Crisis on Infinite Earths", the Arrow series finale showed the aftermath of the event and how "Crisis" changed things in Star City. It was revealed that after the rebirth of the multiverse, Oliver succeeded in undoing the deaths of Tommy Merlyn, Emiko and Moira Queen, and Quentin Lance. The episode also featured guest appearances from The Flash's Barry Allen, Legends of Tomorrow's Sara Lance, and Supergirl's Kara Danvers, who came to Star City to honor their fellow superhero. Aside from them, many others showed up for Oliver's funeral as well, including Anatoly, Thea, Felicity, Nyssa, Talia, Ragman, and Curtis.
So many characters were present at Oliver's funeral that not all were able to have speaking lines. Though Arrow's funeral scene was packed with an impressive number of key Arrow characters, one person was missing: Slade Wilson, aka Deathstroke. Deathstroke did make an appearance through a flashback scene that showed how Oliver averted Moira's death, and though this looked like new footage, Manu Bennett didn't actually film any additional scenes. This was put together through a deleted scene from Arrow season 2.
Bennett never actually came back for the finale, despite his character's importance to the show. As for why he wasn't around, the answer to that isn't clear. In August, Bennett said on Instagram that he hadn't been asked to return. Previously, The CW was told by DC to stop using Deathstroke again, so it could be that this was the reason why he never showed up. However, DC doesn't seem to have rules about Arrow using other versions of Deathstroke, so it's hard to say if this is what happened.
In any case, it is disappointing that Manu Bennett couldn't come back to pay his respects to Oliver, as they certainly had one of the Arrowverse's most interesting relationships. Deathstroke gave Oliver a reason to hate him when he killed his mother, but after the effects of the Mirakuru wore off, Oliver managed to overcome his problem with Deathstroke and forgive him, knowing that Slade wasn't in complete control of his actions in season 2. In the face of everything that had happened between them, the two still had a great deal of respect for each other, and the Arrow series finale could have touched on this relationship by bringing back Slade, even if it was only for a few moments. It's also worth wondering what kind of person Slade would be now, especially since in the post-Crisis Arrowverse, he never actually killed Moira. Unfortunately, this question may never be answered.