Deathstroke was an important character on Lian Yu, but he didn't become relevant to the present-day story until season 2 when he emerged as the main villain. Due to the effects of Mirakuru, Slade went from ally to enemy. After a while, the Mirakuru finally left Slade's system, allowing him to slowly regain his senses.
Following his defeat in the Arrow season 2 finale, Oliver (Stephen Amell) trapped him on the island, only to return to enlist Slade's help in a season 3 episode. That was the last time Deathstroke was seen until the last two episodes of season 5, when Team Arrow had to team up with him to deal with Prometheus (Josh Segarra). After he was finally granted his freedom, Deathstroke appeared in a few episodes of season 6 in Arrow series finale, but this was actually made possible by a season 2 deleted scene, rather than new footage.
So what's the explanation for Deathstroke's absences from Arrow, and why did fans have to wait until season 5 to see him again? The reason is related more to DC's restrictions on the character than it is to Bennett's schedule. According to Arrowverse executive producer Marc Guggenheim, DC stopped The CW from using the character because they had plans of their own for Slade Wilson.
DC actually didn't want Deathstroke to be used again at all, but Guggenheim was able to convince them to make an exception for season 5 and the beginning of season 6 because the story involved Lian Yu, so it was important for Deathstroke to be there. Guggenheim Deathstroke into a two-part story before having to give up the character again. After that, the Arrowverse went back to the days were possible Legion of Doom focused sequel, which also hasn't seen any additional development.
Despite all the past issues sharing the character, it seems like Warner Bros. is finally loosening up on this front, as unique relationship with Oliver Queen, and it would have been interesting to see what Arrow would have done with him if his appearances on the show were much less sporadic.