User blog comment:Tupka217/Christopher Jones teases new comics characters/@comment-5188108-20120828160928/@comment-1038387-20120829143906

It's nothing new; people often point to the "Bat Embargo" of Justice League and Justice League Unlimited, but it's actually far older.

Joker was slated to appear on the Legion of Doom in Hannah-Barbera's Challenge of the Super Friends, but because Filmation had Joker in The New Adventures of Batman at the moment, they couldn't use him. Vice versa for the Riddler; he appears on the Legion of Doom and was off-limits for TNAB (he does appear in the opening credits). Joker didn't appear on Super Friends until TNAB was over, in The Super Powers Team: Galactic Guardians