The Battle Royale mode is said to be in development with Raven Software, which has had a hand in every Call of Duty title since the original Black Ops. The way Polygon worded it makes it seem as though their source indicated that the game's single-player has at least been worked on. They added this same source also told them in place of the story mode would be a battle royale mode.
Dropping the campaign from Black Ops 4 would be a big change, but not entirely unprecedented.
According to the report, Black Ops 4's developers at Treyarch realized a standout single-player campaign couldn't be finished in time for the Oct.12 release. With Black Ops 4, the series may be embracing the growing trend of online multiplayer shooters like Overwatch, Fortnite, and PUBG. However, we don't know whether the development of a campaign has been halted or only postponed until after the game's release.
The first rumbles of this decision came from a report by Polygon, which claims that Black Ops 4 will be the first COD game that skips the single-player campaign.
Instead of a traditional campaign mode, the co-op mode could serve as a replacement option. According to the report, the resources would be shifted to multiplayer and zombies content.
Call of Duty: Black Ops 4 will have a full "Community Reveal" event on May 17th in Los Angeles, California, and will officially release on October 12th, 2018.
Polygon reached out to Activision, and Activision stated, "We don't comment on rumor and speculation".