Out of all the versions of Titans, The Flash, and Batwoman wouldn’t be possible back from 2001-2011 due to how restrictive WB was at the time.
However, there were a few Justice League characters that were able to make it to Smallville, with some having bigger arcs than others, like Green Arrow. From Martian Manhunter to Black Canary, Smallville gave some of these characters a spotlight in live-action before they got their dues in the Arrowverse. There were some big superheroes that Smallville simply couldn’t feature, such as Batman and Wonder Woman, no matter how hard the creators tried. But with characters like The Flash and Green Lantern, they managed to find a grey area that wouldn’t conflict with Warner’s plans (at the time) for the two heroes on the big screen.
While the DC drama had the fourth Flash, Justice Society of America got introduced in a 2-part event. During the first hour of the JSA intro, Clark discovers the Golden Age of superheroes that fought crime long before he was even born on Krypton. Through old footage, Clark witnesses several JSA - and that’s where Smallville introduces the classic versions of Flash and Green Lantern.
In the montage as well as Clark’s visit to the JSA brownstone, Jay Garrick/Flash and Alan Scott/Green Lantern are introduced, played by Billy Mitchell and Doug Pinton respectively. They are seen as being arrested with their fellow while also showcasing Jay’s iconic winged helmet as well as Alan’s Green Lantern ring. At the JSA brownstone, viewers even get to see Alan’s Power Battery next to his nameplate and ring. While Smallville never had Jay and Alan show up again in the show’s 2 remaining seasons, the Golden Age Scarlet Speedster does have a big arc in the Season 11 tie-in comic during the Teen Titans storyline.
While Alan Scott will be getting a bigger spotlight in HBO Max’s Green Lantern series, Jay Garrick continues to be a recurring guest character on The Flash, played by John Wesley Shipp. But thanks to Smallville, those Golden Age heroes got a moment to shine while also allowing a version of The Flash and Green Lantern to that universe. It was also through the Tom Welling-drama that the JSA got some exposure in live-action before shows like Legends of Tomorrow and Stargirl explored the team deeper. So while Flash and Green Lantern characters like Barry Allen and Hal Jordan never came on Smallville, at least the classic Golden Age incarnations got to.