Where did "other sublist fields" go? It was in customization now it's not, my SB has it but prod does not.

I don't even have it in either. I'm also looking for the same thing. I found Customization> Forms > Sublists, but that can only be made from a Saved Search. I don't want it sourcing from a Saved search. I want to manually input the data but don't know how to get this done