Warning: spoilers lie ahead for the second episode of Legends of Tomorrow Season 3, called "Freakshow."
Legends of Tomorrow features the biggest superhero lineup in primetime nowadays, with some of the most entertaining and unlikely comic characters ever to hit the small screen. The third season has sent the crew of Legends on a grand mission to take care of timeline anachronisms throughout history, and "Freakshow" took them back to 1870 to collect a saber-toothed tiger that should not have been alive, let alone in the continental U.S. They ran into P.T. Barnum, played by none other than Titanic alum Billy Zane, and he dropped an unexpected and pretty awesome reference to Superman:
Let me make it very simple for you: become the Man of Steel! That is very catchy, make a note of that.
After the Legends set the saber-toothed tiger loose, P.T. Barnum was in need of a new big attraction for his circus. After seeing Nate use his abilities, Barnum decided that he would get his hands on Nate and use him to wow the audience. Nate was understandably unwilling to help, especially after Barnum had him bound to a wheel and pointed a gun at him. It wasn't until Barnum threatened Amaya that Nate used his abilities and repelled the bullets. In the process, Barnum coined the term "Man of Steel" years before Supergirl would come to Earth-1 as the Girl of Steel.
Of course, we shouldn't take Barnum mentioning "Man of Steel" to mean that Superman has been active on Earth-1 ever since Arrow kicked off the Arrow-verse back in 2012, but the quick name-drop was definitely fun. We may never get Superman or Batman or any of the original Justice League heroes other than the Flash on the Arrow-verse's Earth-1, but we can always count on mentions of anybody and everybody from DC history coming from the time-twisting Legends of Tomorrow. My only disappointment in the Man of Steel mention is that former Superman Brandon Routh wasn't on hand as Ray Palmer to witness it.
That said, Legends of Tomorrow has made more concrete mentions of Superman and Batman in the past. Back in Season 1, Rip Hunter mentioned that he'd seen "men of steel die and dark knights fall," which fans naturally took to mean that Rip had seen versions of Batman and Superman at some point in the timeline on some Earth. The Legends and the various time travelers of the series have made so many changes that it's entirely possible that the Arrow-verse's versions of Batman and Superman simply never existed or blinked out of existence.
This could be Legend of Tomorrow's way of working Superman in without bringing the Man of Steel in himself, and it may be for the best that he doesn't exist on Earth-1. He could totally eclipse the heroes already existing in the Arrow-verse. That said, if Legends of Tomorrow ever finds a way to bring Tyler Hoechlin in as Superman in literally any way, I'll be fine with it. Legends is the show in the Arrow-verse that could get away with bringing in a huge character for a quick appearance for the fun of it. We'll have to wait and see.