The Dark Ages talks about why the game won't include a multiplayer mode like the past few titles in the series.

Instead, it’s keeping both feet planted firmly on the ground and bringing the high-speed, high-skill-ceiling first-person shooter even closer to the scores of Hell’s minions in the medieval-tinged prequel,

Doom: The Dark Ages will be strictly single-player, featuring no multiplayer content at all. The decision to focus solely on single-player was made to allow for unique experiences like the Atlan mech levels. Developers prioritized creating the biggest and best Doom game with challenging combat, giant mech battles, and dragon-riding vehicles.