The animated show Justice League: Unlimited notably featured dozens of both iconic and obscure DC Comics characters, but the popular hero Nightwing somehow only ever got a three second cameo in the whole series. The reason for this comes from a specific policy within Warner Brothers at the time that in today's landscape of cinematic universes seems completely bizarre.
Justice League: Unlimited was the last in a long line of interconnected DC animated television shows, sometimes referred to as the DCAU, or the Timmverse or Diniverse after some of its main creators and architects. This animated continuity began in the early 1990s with the classic cartoon Batman: The Animated Series. It's where this universe's version of Dick Grayson first appeared as Robin, and then in the follwup series The New Batman Adventures graduated to the title of Nightwing. Given the DCAU's roots it seemed as if a character like Nightwing would be a perfect addition to the expanded Justice League roster in Unlimited, but he's only ever seen very briefly in one episode, season 3's ninth episode "Grudge Match".
The lack of Nightwing in the series can be blamed on what was called the "Bat Embargo". In the mid 2000s Warner Brothers was gearing up to reboot their Batman franchise with Batman never shows up in Smallville, it's why he's never even mentioned in Teen Titans, it was one of the factors for George Miller's Justice League: Mortal getting canceled, and it's why Batman characters like Nightwing, Robin, Batgirl, and almost all of his rogue's gallery never appear in either Justice League show.
What makes the embargo especially strange to look back on is the world of superhero entertainment we currently live in today. Worries about oversaturating the market seem to have gone out the window now that, in a post MCU world, the name of the game is vertical integration and for audiences to constantly be taking in new branded content. DC and Warner Bros. especially have changed their tune. Between the DCEU, Joker, and Matt Reeve's film The Batman, there's going to be multiple versions of the caped crusader operating at the same time. The companies are now firmly in the camp of trusting audiences to be able to keep track of the differing versions and accept all of them as their own distinct interpretations of Gotham's Dark Knight.
It stands to reason that if Justice League: Unlimited was made today then DC would have no issues with it utilizing whatever Batman characters the creators would've wanted. It all just came down to the very specific time the show came out, and how much the thinking around superhero media has changed in the fifteen years since it ended.