Will account merging in fortnite come back sometime?
I don't assume no. Many users have bought or hacked accounts in order to bring rare outfits / emotes / etc to their own account.
In order to prevent this from happening again, in my opinion the function was removed again quite quickly (May 2019) after it was published (September 2018).
So: I think this function will no longer exist. Epic Games alone will decide whether she will come back.
Nice day!