Paul Blackthorne's Quentin Lance may have perished in the Arrow season six finale, but he (like other characters who have died or left the show) will turn back up in the series finale early next year, according to TVLine.
The circumstances around Lance's reported resurrection and return to the show remain shrouded in mystery - but considering Arrow will conclude following CW's "Crisis On Infinite Earths" TV crossover which spans numerous realities, anything is possible. The show pulled a similar trick in the season 8 premiere, introducing an Earth 2 version of Tommy Merlyn, another deceased supporting character.
Arrow season 8, the show's final season, is currently airing on CW. The series finale will air in early 2020.