https://www.gamespot.com/articles/no-new-assassins-creed-game-coming-in-2019/1100-6461327/

Ubisoft has revealed that it will not launch a new Assassin’s Creed title in 2019, and instead will support this year’s entry, Assassin’s Creed Odyssey, with additional content designed to keep players engaged in the game. Speaking at GamesCom, Ubisoft CEO Yves Guillemot explained that Assassin's Creed Odyssey's 2018 release was enabled because both 2017's Assassin's Creed Origins and Odyssey were being developed at the same time, and Odyssey will not be immediately followed by another game.

"On Assassin's, we had a game [in 2018] and we have one this year, but we are not going to have a full-fledged Assassin's next year," said Guillemot. "It's just because the team were working separately, so we have two games now, one year after the other. But next year you're not going to have a fully fledged one."