https://wccftech.com/call-of-duty-black-ops-4-a-campaign/

Call of Duty Black Ops 4 is the first entry in the Black Ops series to lack a traditional campaign, but it seems like the game did feature it during development, only to be scrapped later.

A few hours ago, a fired Treyarch QA tester called CallOfNobodyCares revealed on Reddit that Call of Duty Black Ops 4 campaign has been in the works until 2017. It was designed to be a 2v2 co-op race to finish each mission. 5 missions were apparently complete, but Treyarch later decided to remove the campaign altogether, as they wanted to add more mission but were unable to due to time constraint. Blackout and Zombies development started right after the campaign was scrapped.