UPDATE 3rd July 2019: Split-screen returns to Blackout 9th July, Treyarch has announced.
In a tweet, the developer said split-screen returns in all Blackout modes as part of the Operation Apocalypse Z update, which goes live on 9th July.
Alcatraz will return to #Blackout on all platforms on July 9th, along with a dark new twist coming to Alcatraz in Operation Apocalypse Z.— Treyarch Studios (@Treyarch) July 2, 2019
Splitscreen co-op will also be restored in all Blackout modes with our July 9th update alongside new Blackout map updates.
ORIGINAL STORY 29th July 2019: An odd thing happened to Call of Duty: Black Ops 4 this week: Blackout split-screen was removed from the game and developer Treyarch didn't tell anyone.
Thankfully, a rep for Treyarch chimed in on reddit. They confirmed Treyarch has "temporarily" disabled split-screen "as we evaluate some known issues".
"We'll share updates in the future after investigating further."
Then: "Currently disabled and under investigation/evaluation."
There's no word on when split-screen will return.
Perhaps it's apt Treyarch quietly pulled Blackout split-screen from Black Ops 4, given the studio quietly added it to the game in the first place.
Back in October 2018 I reported Black Ops 4 had split-screen, but Treyarch didn't shout about the feature in the run up to the game's release.
Split-screen is (was?) available for two in competitive multiplayer, and for up to four players in Zombies. In Blackout, split-screen performance was always pretty rough. Back at launch, I played a game of the battle royale with Digital Foundry's Tom Morgan and he mentioned a significantly lower framerate, graphical issues and a general sluggish feeling compared to the razor sharp default experience. Based on player reports, Blackout split-screen hasn't improved since.