Brandon Routh's Superman in the Crisis on Infinite Earths Arrowverse crossover will be the Kingdom Come version of the character. Even though Routh previously played Superman in Superman Returns in 2006, he has spent the last several years playing a different DC hero as part of the Arrowverse. He's starred in Arrow and Legends of Tomorrow as Ray Palmer (aka The Atom), but is now getting the chance to suit back up again as Superman.
It was revealed earlier this week that Routh would be playing two parts in the 2019 Arrowverse crossover event. He will continue playing The Atom for the five-hour event Crisis on Infinite Earths, but he will also play Superman again. At the time, it was unclear if he would play the Superman Returns version of the character or not. Many fans hoped that would be the case, as it would be a clever way to make the film a parallel universe to the Arrowverse - but he's playing a different version instead.
During the Legends of Tomorrow panel at San Diego Comic-Con 2019, Routh walked out to the panel and revealed that he will play the Kingdom Come version of Superman in Crisis on Infinite Earths. Routh wore a shirt with the altered Superman logo on it to signify his new role, which you can see below thanks to @Pagmyst.
The Kingdom Come version of Superman comes from the Kingdom Come Elseworlds mini-series created by Mark Waid and Alex Ross. In this story, Joker kills the entire Daily Planet staff, including Lois Lane. He is arrested for the killings and put on trial, but he is killed by a new hero, Magog. Magog is praised for his heroic deed, which Superman does not agree with, and spends the next decade away from humanity in the Fortress of Solitude. He is brought back into the superhero lifestyle by Wonder Woman and reforms the Justice League. The two become romantically involved and are expecting a child when the mini-series concludes.
It is unclear how much of this backstory will actually be addressed in Crisis on Infinite Earths, or if this means that Wonder Woman is on her way to the crossover, too. But, it is nice to have clarity on the version of Superman that Routh will be playing. He previously said that he had mixed emotions about coming back to the role, but playing a completely different iteration may have helped convince him to suit back up. After offering fans a tease of him wearing the logo, it hopefully won't be too long before an official look at him in the Kingdom Come Superman suit is released.