As said, Kopa is only in books and was ignored, when they made second movie. So he is not really part of official movies.

But fans did not want to forget him so soon. Especially since both presentations, in TLK1 and TLK2 are different, including cubs look different, they made their own theories. These theories also try to fill in some other plot holes and explain some stuff better than it was done in movie. So you have many stories about Kopa floating around internet.

Everyone has their own theory, some people say he died and some say he survived. Some poeple say he died so that's why he wasn't in the sequal, others say that he survived but ran away and that's why he wasn't in the sequal. Some people think he was born after Kiara, some say he joined Zira's pride, and others say he didn;t exist at all. There is no technical answer to this, only theories, so feel free to come up with your own

Kopa did not die because he exists only in the few books in the book universe of The Lion King, and in them nothing fatal happens to him. What happened to him seeing to the movies is that he got completely ignored and denied by all film makers.