Warning: The following contains SPOILERS for The Flash season 8, episode 1, "Armageddon, Part 1."
Brandon Routh gave a subtle shout-out to his big break as Superman with one pose in The Flash season 8 premiere. While Routh first found fame playing the Man of Steel in Superman Returns and recreated the role during the CW's ambitious Crisis on Infinite Earths event, he is best known to Arrowverse fans for playing Dr. Ray Palmer (aka The Atom) in Arrow season 3 and Legends of Tomorrow.
It was in this capacity that Routh joined the ensemble of Arrowverse heroes appearing in the Armageddon crossover event, reprising his character for the first time since Ray and his new wife, Nora Darhk, took their leave from the franchise. The plot of The Flash season 8 premiere finds Ray back in Central City for the Central City Tech Con, appearing as a special legacy honoree. He shows up on Barry and Iris' doorstep and is offered their guest bedroom after the convention hotel is overbooked. This gave Barry and Ray a chance to catch up, with the later explaining that he was focusing on his relationship with Nora and "hadn't worn the Atom suit in ages."
Despite this, Ray still keeps his costume handy and is ready to step back into the action when the alien villain Despero shows up at the Central City Tech Con intent on calling out The Flash for a fight. When it becomes clear that Barry can't handle Despero alone, Ray informs Chester P. Runk and Iris West-Allen that he is going to lend The Flash a much-needed helping hand. With that, Ray runs off, loosening his tie and ripping open his button-down shirt in the same familiar way that Clark Kent has done across numerous Superman properties.
Logically, there was no reason for Ray Palmer to do this. The Atom Suit was usually shown as fitting itself over whatever else Ray Palmer might be wearing whenever he needed to become the Atom, much like Tony Stark's Iron Man armor in the Marvel Cinematic Universe, and beyond. The suit is all about efficiency, as Ray usually keeps the costume shrunk down to the size of a Hot Wheels toy car, so that he can carry it with him more easily. The only reason for Brandon Routh to have assumed this classic superhero pose was as a tribute to his time as Superman.
Unfortunately, those fans hoping that Routh might once again reprise his role as the Man of Steel of Earth-96 as part of The Flash Armageddon crossover event are likely to be disappointed. There has been nothing in any of the preview materials or the story so far to indicate that any version of Superman will be showing up in Armageddon, or that The CW might be considering a spinoff show devoted to Brandon Routh's Superman. However, it is nice that the actor was able to give a knowing nod and a wink to the role that made him famous as part of the event.