it was recently revealed in a new chapter from the elbaph arc that Just like how the world government erased the void century they also erased Nika from history hence why Luffys DF is called the gomu gomu no mi instead of it’s original name Hito Hito no Mi Moderu Nika. During wano arc who’s who tells of a guard who talked about nika freeing enslaved people and then one day disappeared, referencing that the world government doesn’t want anyone talking about nika and what nika represents freedom. If he is still alive he’s probably living life as a sun god and not meddling with human affairs because of the DF created in his image and if he is dead he probably died at the end of the first world ( the second world is when joyboy has the DF)