At what point did storytelling shift from legend to fiction proper?

legend is always legend

multiple ansewrs kinda

appropriation of legend, rather than reenactment of legend, with classical athenian theatre

decline of need to appropriate legend / situate plots within familiar shared cultural myth setting happens in bits and bobs all over the place, even extremely ancient, but probably good case for it becoming mainstream in renaissance and reformation, and even then it's extremely common

modern narrative prose fiction proper around the same time, e.g. cervantes sometimes called first novel, maybe bit later, enlightenment, but also lots of arguments are made that the truly "modern" "psychological" "novel" is very late, 19th century even, often ascribed to victorian

benedict anderson's print culture creating shared cultural conception of a "space where stories can take place" similar to old legend, now it becomes familiar locale bounded by print culture and nation state units

also shift from mythic/legendary to (various definitions of) "secular" world-conception is prominent for a lot of tinkers

I believe you have an assumption built into that question, son.