It seems to me that within the past few versions of Minecraft, terrain was extremely customizable. I distinctly remember that the sea level was adjustable, and you could decide whether overworld oceans were made of water or lava, or both. Now it's all gone. Was this all a hallucination or did these options get removed for some reason?
The Custom World Generator was not included in the changes to terrain and biome generation that made 1.13 possible. It is also an extremely outdated system that looks very ugly and obnoxious to use. It was removed simply because allowing it to remain caused Minecraft to crash hard (much like using mods for the wrong version of Minecraft). The code stuff is still there, it's just completely walled off so that users can't cause Minecraft to crash (at least for these specific reasons).
It is currently being worked on, but the intention is not just to preserve what we had originally. The options that were formerly present will be expanded to include more control over more things and reorganized to be much more intuitive. The entire system is also getting a facelist from "list of textboxes" to what can be likened to a graphical rendering platform. In the player inventory, you know how there's an image of your character that follows the mouse as you move it? That's sort of the look they're going for, except that the picture you will see would be an example of the world you're creating.