of videos online showing the hints of his existence, and a lot of fans explain his disapperance as a murder (Primarly blaming Zira the villian from TLK 2) using that to explain Simbas distrust for the outlanders and his over protectiveness of Kiara.
But I have my own headcanon, lions reach sexual maturity at 2-3 years of age, when that happens they are expelled from their pride and have two options, either they can grow and come back to challenge their father (or whoevers leading at the time) for leadership or they can start their own pride, upon reaching this age Kopa left but fell in love with a male lion (Homosexuality is very common in lions) and decided to start a new pride taking in orphaned cubs and rogues, and I'm not saying he never returned cause Simbas homophobic, again being gay is normal for lions, but Kopa knew how hard his father had fought to reclaim their home and knowing that he could never provide an heir and that his mother was pregnant with a new cub, he left to

let the new cub take up the mantle of leader.

I'll admit I never grew up with Kopa like others did but I just think it'd be cool for old fans to see him, maybe make a movie where Kion (Simbas youngest son) finds him lying injured and brings him home where Nala and Simba must reveal to Kiara and Kion who that lion really is.

Also the lion kings style is waaaaay more difficult to emulate than I thought.