Minecraft wiki entry for Custom, associated text: "Would you really not consider holding off 1.13 until customised worlds are finished? Seems pretty crazy to set a precedent for the chance of losing features for several updates.
We came so close to having access to customising structures in world gen this update too, a shame that hasn't made the cut" – u/DaUltraMarine
"No, they will come back but completely different from before." – u/_Grum on Reddit, May 15, 2018