https://www.eurogamer.net/relic-leaves-dawn-of-war-3-behind-as-it-moves-on-to-new-projects
Dawn of War 3's loyal player base was deemed too small to justify further investment in the game, with Relic and Sega chiefs expressing doubt that updates could turn things around. The decision to leave Dawn of War 3 behind means no new races will be added to the game, nor any expansions released, which will come as a disappointment to fans. Dawn of War 3 launched with just three races (Space Marines, Orks and Eldar) and the expectation was more would be added post-launch.
We first heard Relic had considered Dawn of War 3's future late in 2017, but today Relic, via publisher Sega, issued Eurogamer a statement addressing the issue. The statement, below, is ambiguous in that it suggests Relic is still working out Dawn of War's future, but we understand there are no plans for new contente.