Superman subtly confirms the existence of the DCU Flash

Superman opens with a text revealing that this universe is familiar with metahumans because of roughly 300 years of exposure. This means that The Last Son of Krypton is not the hero that inspires other heroes to come out. This aspect is in firm contrast to the DCEU, where the Man of Steel is the first hero to go public, inspiring Wonder Woman, The Flash, Shazam, and Aquaman to be public as well.

The existence of metahumans also suggests that the heroes might have teamed up in the past. One of the common examples of a super team in the DC comics lore is the JSA, or Justice Society of America, the precursor to the Justice League. The history of the JSA is hinted at by Superman through a mural inside the Hall of Justice. One of the figures that's drawn in the mural is Jay Garrick's The Flash.

Superman's Hall of Justice mural explained:

The Hall of Justice is the headquarters of the Justice Gang, which is revealed to be sponsored by Maxwell Lord. In that scene, Lois visits Guy Gardner's Green Lantern, Michael Holt's Mister Terrific, and Kendra Saunders' Hawkgirl to convince them to help Superman, who is imprisoned inside Lex Luthor's secret pocket dimension.

The biggest feature of this scene is the mural inside the Hall of Justice on the wall behind Lois. This mural describes the history of the DCU and its countless superheroes over 300 years, meaning some of the heroes drawn in the mural might've influenced Superman. But one of the noticeable heroes in the murals is none other than The Flash.

However, the Flash drawn in the mural is not the Flash we are familiar with, i.e., Barry Allen or Wally West. The one in the mural is Jay Garrick, the very first Scarlet Speedster, before Barry Allen. In the comics, Garrick appeared first in January 1940. Barry was introduced in October 1956 as a scientist who gained his powers from a lightning strike and christened himself as "The Flash" after his favorite comic book hero (Jay Garrick version).

James Gunn discusses the easter eggs in Superman:

Early this month, James Gunn attended a fan event at Leicester Square in London, in which 30 minutes of the movie were shown. There, the Superman director teased what we can expect in terms of easter eggs:

"This is the most Easter egg-laden movie I've ever made. I have a lot of easter eggs in this movie. I mean every character in the movie is practically from the comic books. Everyone in Lex's control room- all those people are characters from the comics...Every street in the movie is named after a comic book writer or artist."

Then, teasing the Hall of Justice scene, Gunn continued:

"Most of the businesses are either named after either a comic book writer or artist, or they are named after businesses that exist within the DC Universe, on the page. Later on, there's an iconic scene with a huge, huge mural in the background that tells you the story of metahumans in the DCU and that's going to be really, really fun when we release those images to the public. So the stuff goes deep, it's really really fun and it was really fun creating the world. That was one of the reasons why I like DC a lot because it isn't New York and Los Angeles, it's Gotham and Central City and Evergreen. It's really creating another world, with other rules and other things that are common."